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第二十一章 虚拟式
一、虚拟式
英语中有三种语式：陈述语式、祈使语式和虚拟语式。
A.陈述语式
表示谓语动词所表达的动作或状态是符合客观现实的，也就是说把动作或状态当做事实表达出来。例如：
Flowers blossom in spring.
The yellow leaves are falling in the autumn wind.
The moon is peeping through the windows at the sleepless girl.
B.祈使语式
表示号召、命令、请求、劝告、警告、禁止等。例如：
Let us help you.
Do drop in to see us.
Keep off the grass.
C.虚拟语式
虚拟语式把动作当做一种只存在于讲话人想象中的“假设”或“推测”，而不是当做客观现实中的真实事件。它表达的是怀疑、忧虑、推测、假设、想象或祝愿等。例如：
Mother told me to put on my sweater lest I should catch cold.
二、虚拟语式在条件句中的运用
英语中的条件句一般有两种：真实条件句和非真实条件句，虚拟语式用在非真实条件句中。非真实条件句包括虚拟条件句、推测条件句和错综时间条件句等。
（一）虚拟条件句
虚拟条件句可分为两类：一类是叙述与现在事实相反的情况，一类是叙述与过去事实相反的情况。其构成如下(would有时可换用should, could或might)：
A.与现在事实相反:
条件从句--If I (we, you, he, she, it, they) +动词过去式(be动词的过去式一律用were
结果主句-- I (we, you, he, she, it, they)+would+动词原形
B.与过去事实相反:
条件从句-- If I (we, you, he, she, it, they) +had+过去分词
结果主句-- I (we, you, he, she, it, they) + would +have+过去分词
C.与将来事实相反:
条件从句--
A式：一般过去式
B式：were＋不定式
C式：should+动词原形
结果主句--would (should) + 动词原形
例如：
He wouldn't feel so cold if he were indoors.
他要是在室内就不会觉得冷了。（与现在事实相反）
I am sorry I am very busy just now. If I had time, I would certainly go to the movies with you.
我很抱歉，现在很忙。如果有时间，我肯定陪你去看电影了。（与现在事实相反）
The conference wouldn't have been so successful if we hadn't made adequate preparation.
如果我们不做充分的准备，会议是不会开得这么成功的。（与过去事实相反）
The flood might have caused great damage to the people if we didn't built so many reservoirs.
倘若我们没有修建这么多的水库，洪水就会使人民遭受巨大损失了。（与过去事实相反）
Note:
在同一个句子中，仅有一部分表示真实条件，另一部分表示非真实条件，不然就会破坏句子的一致性。例如：
If he is here, he would not let the matter end this way.（误）
If he were here, he would not let the matter end this way.（正）
If I had the book, I will lend it to you.（误）
If I had the book, I should lend it to you.（正）
另外，条件从句中不可用would,否则就是语病。例如：
If he would have come yesterday, I should have told him.（误）
If he had come yesterday, I should have told him.（正）
should＋动词原形比were+不定式表示的实现的可能性大。
（二）推测条件句
对未来的事进行推测，在讲话人心目中，虽不认为推测条件的实现违背事实或绝对不可能，但却认为可能性是极小的。这种怀疑和推测要用虚拟语式表示，其构成形式如下：
条件从句--1. If I (we, you, he, she, it, they)+动词过去式(be的过去式一律用were)
2. If I (we, you, he, she, it, they) + should + 动词原形
3. If I (we, you, he, she, it, they) + were + to +动词原形
结果主句--I (we, you, he, she, it, they) + would+动词原形
例如：
If I were to see him, I would speak to him about it.
如果我见到他，我会跟他谈这个问题。
（三）错综时间条件句
在错综时间条件句中，虚拟条件从句和主句动作发生的时间不一致，因此，主句和从句的谓语动词要根据各自所指的不同时间选用适当的虚拟语式形式。例如：
Had I taken my umbrella with me when I came out this morning, I should not be wet now.
如果我早上出门时带了伞，现在就不会淋湿了。
If you had spoken to him last time you saw him, you would know what to do now.
如果你上次看到他时跟他说一下，现在就知道怎么做了。
If I were you, I wouldn't have missed the film last night.
如果我是你，我就不会错过昨晚那部电影。
If the ship had left at noon, it would be passing through the canal now.
船要是在中午启航的话，此刻该在通过隧道了。
A wise man wouldn't have done that.
三、无“条件”的虚拟语式句子
这里的所谓无“条件”，指的是句子表层没有通常使用的if或I wish等引起的表示条件的句子，但其深层结构或是上下文中还是有条件的。这种条件可以用介词、形容词、动词、分词、不定式、定语从句或上下文等表示出来。表示无“条件”的词有with, without, but for, or, but, otherwise, even, in case of, what if等。这种句子往往是有主句而无从句。
例如：
But for your help we couldn't have succeeded.
They wouldn't have reached the agreement so easily without that common ground.
上面两句中的bur for 和without 可以用If it were not for来代替。
Under such circumstances I would probably have done the same.
She wasn't feeling very well. Otherwise she wouldn't have left the meeting so early.
I would do so in your place.
With better equipment, we could have done it better.（介词短语）
She said she would have come to the party, only she had an urgent matter to attend to.（副词＋句子）
To talk with him, you would know he is a frank man.（不定式短语）
To have studied harder, you would have passed the exam. (= If you had ..，不定式完成式)
The same thing, happening in war time, would amount to disaster.（分词短语）
On being shown this book, no one would believe that it was written by a ten-year girl.（介词+动名词）
In doing so, he might forgive you. （介词＋动名词）
A man who stopped working would amount to nothing.（定语从句）
He would be a foolish man who should venture that.（定语从句）
You should never join th club, it would do you no good.（上下文）
Give him an inch, he would want a foot.（上下文）
It would be wrong not to take this matter into consideration. (=If you did not take…)
They would have paid attention to a nice girl. (= If she had been…)
Such mistakes could have been avoided. (= If one had been more careful)
She would have acted as you did. (=If she were you)
四、but+表示条件的结构
Would+动词原形+ but (except)＋现在时陈述句（陈述语式）→ 表示同现在事实相反的假设。
Would have +过去分词＋but (except)＋过去时陈述句（陈述语式）→表示同过去事实相反的假设。
例如：
But for her guidance, we should fail.
I would have gone with her but I was too busy.
A:“Why didn't Henry come?”
B:“He would have come except she didn't have time.”
She would gain weight but she doesn't eat enough.
Note:
比较下面两句：
She would have come to see you but she was ill that day.
She would have come to see you but that she was ill that day.
第一句中的but则并列连词，引导的是并列句，but意为“但是”；第二句中的but是介词，意为“除去”。引导条件状语从句。but既可以表示虚拟条件，也可以表示真实条件，但but that只能表示虚拟条件。例如 ：
It never rains but it pours.
She would have bought the book but that she had no money then.
but that结构可以放在句首，并可换为but for+名词结构。例如：
But that I saw it, I couldn't have believed it.
But that it snowed heavily, they could have arrived there earlier. (= But for the heavy snow)
五、虚拟语式在某些从句中的运用
（一）动词wish后的宾语从句用虚拟语式
动词wish后的宾语从句用虚拟语式，与现在事实相反，用were或动词的一般过去式；与过去事实相反，用had+过去分词或would /could+have +过去分词；表示将来没有把握或不太可能实现的愿望，用would / should (could, might)＋动词原形。例如：
I wish she were here.
I wish she had taken my advice.
I wish you would go with us tomorrow.
A:“ You have made some mistakes.”
B:“ I wish I didn't make mistakes every day.”
A:“What would you wish to do if you were a college student again?”
B:“ That's very hard to say, but I wish I had studied psychology.”
Note:
比较下面几个表示未实现的愿望的句子：
I should like to have taken his advice.
我现在真希望（那时）采纳了他的建议。
I should have liked to take his advice.
我本希望采纳他的建议的。
I should have liked to have taken his advice.
（意同上面第二句，但结构笨重）
（二）if only 和would that 后用虚拟语式
if only 和would that 后用虚拟语式，谓语动词用一般过去时表示现在没有实现的愿望，用过去完成时表示过去没有实现的愿望，常译为“要是…就好了”。例如：
If only she had known where to find you.
If only I could speak several foreign languages!
Would that I were young again.
would that 实际上是I (we) would的简化，I would相当于I wish.例如：
I would I were a bird and could fly freely in the sky.
We would that we had seen through him earlier.
Note:
O that 和 Would to God是would that的变体。例如：
O that money grew on the trees.钱长在树上就好了。
Would to God she would return safely.但愿她平安归来！
（三）在suggest, order, demand等后要用虚拟语式
在propose, command, request, desire, insist, require, decide, promise, arrange, ask, move, consent, determine, recommend, advise, intend, beg, resolve, argue, maintain, deserve, prefer, decree, vote, urge, instruct, advocate, persuade, dictate, legislate, sentence, object, expect, direct, appoint, provide, specify, persist, permit等意念动词、祈使动词或态度动词后的宾语从句中，要用虚拟语式，这时，谓语动词用动词原形或should+动词原形。例如：
He determined that she (would) go at once.
They intended that the news (should) be suppressed.
I prefer that you not eat the meat.
She insisted that the seats (should) be booked in advance.
The workers demanded that their wages (should) be raised by 10 percent.
They requested that the (should) sing a song.
He advised that the doctor (should) be sent for.
The committee decided that no one should be admitted without a ticket.
Sometimes she would suggest that I should be saving some of the money.
The examination instructions ask that the students (should) not use a red pen.
It was arranged that she leave the next week.
Note:
When he was asked why his friend had not accepted the offer, he suggested that his friend's pride prevented him from accepting.
句中suggested不作“建议”解，而是作“暗示”解，故用陈述语式。如果上述动词作其他意思解释，或者说话者认为所讲的事是事实，要用陈述语式。比如，当suggest作“暗示、表明”解， insist不表示“坚决要求”，而作“坚决认为”解，其后的从句要用陈述语式，不用虚拟语式。再如：
He insisted that he was not involved in the case.
When asked why he declined our invitation, he suggested that he was not on good terms with the director.
（四）在advice, demand, order等名词后要用虚拟语式
在advice, demand, order等名词后的主语从句和表语从句中用虚拟语式。常用的这类名词有：advice, demand, order, necessity, resolution, pay, decision, proposal, requirement, suggestion, idea, recommendation, preference, request, plan, motion等。这时，谓语用动词原形或should+动词原形。例如：
The demand is that manuscripts (should) be written on one side only.
He issued the order that the troops (should) withdraw at once.
（五）在important, necessary等后用虚拟语式
形容词important, necessary, imperative, natural, urgent, essential, appropriate, desirable, insistent, obligatory, probable, vital, advisable, better, keen, preferable, possible, incredible, funny, concerned, ridiculous, unthinkable, 以及短语no wonder, a pity, not right等可以构成“it is +形容词（名词）+that”句型，其后的句子要用虚拟语式，为(should) be型。例如：
It is necessary that some immediate effort (should) be made.
必须立即采取行动。
It is essential that the program (should) be loaded into computer.
把程序输进计算机非常必要。
Note:
It is amazing (odd, strange, surprising, wrong, astonishing, inconceivable, a pity, a shame, a thousand pities, to be regretted)、 I am surprised (sorry) 和I regret等结构后的that 从句中要用should，意为“竟然”，表示说话人的惊异、懊悔、失望等情感。例如：
It is amazing that he should have learnt so much in such a short time.
I am surprised that he should have been so rude to you.
I regret that I should have forgotten to tell you the fact.
It is a pity that Henry should be so careless.
It is a shame that he should have done such a thing.
有时候，可以将表示情感等的句子前一部分省略，只保留that 以后的部分，成为虚拟式的感叹句。例如：
That he should miss such a golden opportunity! (= It is a pity that…)
That they should resort to violence! (= I am surprised that…)
（六）It is (high, about) time…句型用虚拟语式
It is (high, about) time…句型要求用虚拟语式，谓语动词用过去式，指现在或将来的情况，表示“早该干某事而已经有些晚了”。例如：
It is time that we went to bed.
It is time that somebody taught you to behave yourself.
这种结构中的过去式也可改为should+ 动词原形（用should时，不能将其省略），但不如用过去式普通。例如：
It is about time we should go to bed.
It is time she should wash her clothes.
I suppose it's almost time we were leaving. (should be leaving)
上述两种形式均可用It is time for sb. to do sth.代替。例如：
It is about time for us to go to bed.
It is time for her to wash her clothes.
Note:
下面一句通常只用was,不用were:
It's time I was in bed.
另外，这结构中用about或high是为了缓和或加强语气。
（七）在for fear that, in order that, so that, lest 引导的目的状语从句中
在for fear that, in order that, so that, lest 引导的目的状语从句中，从句谓语动词用should(could, might)＋ 动词原形。在以let 引导的从句中，谓语动词用should+动词原形。例如：
He reminded her twice of it lest she should forget.
I will not make a noise for fear (that ) I should (might) disturb you.
I did this so that I might have a couple of weeks to prepare my paper.
Note:
若in order that 和so that 前的主句是现在时，其后的从句有时亦可用can, may +动词原形。例如：
I didn't mean that you should come so early.
He began to think that we shouldn't have waited so long.
They promised that we should have a good rest after the work.
（八）marvel等动词后，从句用should＋动词结构
marvel, rejoice, wonder, regret等表示强烈感情变化的意念动词，用于否定句、感叹句和疑问句中的think, suppose以及hate可以表示意外地可喜、可惊、可悲、可疑等情况，其宾语从句用should＋动词结构，一般不用be型结构。例如：
I marvel that he should be able to do so much in such a short time.
I regret that you should consider it a burden to do it.
Do you think it proper that she should say that ?
I never thought Helen should be so vain and selfish.
比较:
I hate (it) that you have to endure it.（事实）
I hate that you should consider it a burden to do it.
We rejoice (to learn) that you have come back.
I wonder that she should have given up the job.
（九）imagine等动词后用虚拟句式
imagine和suppose (supposing)可以表示现在或将来非现实情况的意想、态度（用were型结构），也可以表示对过去非实际情况的意想（用had＋过去分词结构）。例如：
Imagine that we were on an island now.
Suppose you were laughed at by those shallow people.
（十）以as if (as though)引导的方式状语从句或表语从句，用虚拟语式
以as if (as though)引导的方式状语从句或表语从句，有时用虚拟语式，从句表示与现在事实相反，谓语动词用一般过去时；表示与过去事实相反，用had＋过去分词；表示将来的可能性不大，用would (might, could) ＋动词原形。例如：
He talks as if he knew all about it.
She looks as though she were sick.
He talks about pyramids as though he had seen them himself.
I feel (felt) as if we had known each other for years.
It looks as if it might rain.
但as if (as though)后面的从句有时也可以用陈述语式，这是因为从句中的情况往往是可能发生的或可能被设想为真实的。例如：
It looks as if our side is going to win.
比较：
He walks as if he is drunk. (= He is probably drunk.)
He walks as if he were drunk. (= He walks as if he were drunk, but he is not.)
Note:
注意 "It isn't as if+虚拟式句子"的译法：
It isn't as if he were poor.
直译：他不像穷的样子。
意译：他才不穷呢。（或：他又不穷！）
Why id you invite him to the symposium? It was not as if he were an expert?
你为什么邀请他参加研讨会？他又不是什么专家！
It isn't as if he had claimed to be the owner of the property. In fact he admitted that the property belonged to his uncle.
他又没有声称自己拥有那些财产呀！事实上，他承认那些财产属于他叔叔。
（十一）在though, whatever 等连接代词或副词引导的状语从句中
在though, whatever, if, so long as, whoever, however, whichever, no matter what (who, how)等连接代词或副词引导的状语从句中，用虚拟语式。这时，谓语动词用动词原形，表示推测，让步等含义，但主句一般用直陈语气。例如：
Though everyone desert you, I will not.
Whatever be his defense, we cannot tolerate this disloyalty.
If the rumour be true, we'll have to take other measures.
So long as a volume hold together, I am not disturbed as to its outer appearance.
Whether he be right or wrong, he'll always go his own way.
Be it true or not, I will see myself.
Happen what may, we will not give in.
Come what may, we're not going to make any concessions to his unreasonable demands.
Be the weather what it may, we will leave tomorrow.
Note:
when, granted that等引导的让步状语从句有时亦用虚拟语式。例如：
She walks when he might take a taxi. (=though)
Granted that you had been drunk, there is no excuse for you to do that.
（十二）expect等动词的否定式或疑问式后
expect, believe, suspect, dream等动词用于否定句或疑问句时，其后的宾语从句一般用should ＋ 动词，表示惊讶、惶恐、怀疑、失望、不满等情绪。注意，这种结构中的should不可省略。例如：
He just couldn't believe that his hometown should have gone through such great changes.
（十三）would rather, would sooner,等后的句子
would rather, would sooner, had better, would as soon, just as soon后的句子用虚拟语式。
A)接动词原形，指现在或将来时间，表示一种主观愿望或选择。例如：
I'd rather do it today.
He would sooner die than surrender.
He'd rather not go dancing tonight.
B)接不定式完成式，表示过去的某种选择不恰当。例如：
I'd rather not have told her the news. She is such a gossip.
I'd rather have stayed at home than went to the dull film.
C)接从句，用虚拟语式，为一般过去时，表示一个现在或将来的愿望；用过去完成时，表示一个过去的愿望。例如：
I'd rather you paid the money yourself。（现在）
I would sooner she painted the wall green next time.（将来）
I'd rather she hadn't done that.（过去）
I'd soon you didn't do anything about it for the time being.
（十四）would rather …than 和would…rather than后
would rather …than 和would…rather than后的句子用虚拟语式，这两个结构中than后的从句要用should＋动词原形。例如：
I would rather die than (that) he should know the secret.
He'd do anything rather than (that) he should live with such a shameless woman.
（十五）would, should, could和might用于虚拟语式中
would, should, might和could均可用于虚拟语式中，但在含义和用法上有些差别：
A)第一人称可用should或would，其他人称多用would。
B)would 可以表示情态意义，表示主语的意想和意图，通用于所有人称，should表示意愿和意见时，只用于第二、第三人称。
C)could可以表示“能力”，指由知识、技能、体力等所产生的能力。
D)could可以表示“允许，许诺”，相当于would be allowed to, 但might则无此义。
E)表示“可能”时，could和might可以互换，但在否定句中多用could。
F) ought to表示推断和理应如何，有时亦用must对过去表示推断。例如：
I would lend him the money if he asked me.（意愿）
If he had time, he should do it.（意愿）
If he forgot to come, you could go instead.（许诺）
If she started early, she ought to be here by now.（推断）
If you had been here this morning, you must have seen the famous writer.（推断）
六、条件句及虚拟式的其他表示法
（一）so (as) long as
so (as) long as 表示一个真实条件，谓语动词用陈述语式。
例如：
You may borrow the book so long as you return it in one week.
（二）on condition that
on condition that 意为“在…条件下”，它所引导的从句谓语动词可以用陈述语式，也可以用虚拟语式，但均表示真实条件。
例如：
The guards allow the tourists to go into the woods on condition that they do (should do) no harm to the animals
一、虚拟式
英语中有三种语式：陈述语式、祈使语式和虚拟语式。
A.陈述语式
表示谓语动词所表达的动作或状态是符合客观现实的，也就是说把动作或状态当做事实表达出来。例如：
Flowers blossom in spring.
The yellow leaves are falling in the autumn wind.
The moon is peeping through the windows at the sleepless girl.
B.祈使语式
表示号召、命令、请求、劝告、警告、禁止等。例如：
Let us help you.
Do drop in to see us.
Keep off the grass.
C.虚拟语式
虚拟语式把动作当做一种只存在于讲话人想象中的“假设”或“推测”，而不是当做客观现实中的真实事件。它表达的是怀疑、忧虑、推测、假设、想象或祝愿等。例如：
Mother told me to put on my sweater lest I should catch cold.
二、虚拟语式在条件句中的运用
英语中的条件句一般有两种：真实条件句和非真实条件句，虚拟语式用在非真实条件句中。非真实条件句包括虚拟条件句、推测条件句和错综时间条件句等。
（一）虚拟条件句
虚拟条件句可分为两类：一类是叙述与现在事实相反的情况，一类是叙述与过去事实相反的情况。其构成如下(would有时可换用should, could或might)：
A.与现在事实相反:
条件从句--If I (we, you, he, she, it, they) +动词过去式(be动词的过去式一律用were
结果主句-- I (we, you, he, she, it, they)+would+动词原形
B.与过去事实相反:
条件从句-- If I (we, you, he, she, it, they) +had+过去分词
结果主句-- I (we, you, he, she, it, they) + would +have+过去分词
C.与将来事实相反:
条件从句--
A式：一般过去式
B式：were＋不定式
C式：should+动词原形
结果主句--would (should) + 动词原形
例如：
He wouldn't feel so cold if he were indoors.
他要是在室内就不会觉得冷了。（与现在事实相反）
I am sorry I am very busy just now. If I had time, I would certainly go to the movies with you.
我很抱歉，现在很忙。如果有时间，我肯定陪你去看电影了。（与现在事实相反）
The conference wouldn't have been so successful if we hadn't made adequate preparation.
如果我们不做充分的准备，会议是不会开得这么成功的。（与过去事实相反）
The flood might have caused great damage to the people if we didn't built so many reservoirs.
倘若我们没有修建这么多的水库，洪水就会使人民遭受巨大损失了。（与过去事实相反）
Note:
在同一个句子中，仅有一部分表示真实条件，另一部分表示非真实条件，不然就会破坏句子的一致性。例如：
If he is here, he would not let the matter end this way.（误）
If he were here, he would not let the matter end this way.（正）
If I had the book, I will lend it to you.（误）
If I had the book, I should lend it to you.（正）
另外，条件从句中不可用would,否则就是语病。例如：
If he would have come yesterday, I should have told him.（误）
If he had come yesterday, I should have told him.（正）
should＋动词原形比were+不定式表示的实现的可能性大。
（二）推测条件句
对未来的事进行推测，在讲话人心目中，虽不认为推测条件的实现违背事实或绝对不可能，但却认为可能性是极小的。这种怀疑和推测要用虚拟语式表示，其构成形式如下：
条件从句--1. If I (we, you, he, she, it, they)+动词过去式(be的过去式一律用were)
2. If I (we, you, he, she, it, they) + should + 动词原形
3. If I (we, you, he, she, it, they) + were + to +动词原形
结果主句--I (we, you, he, she, it, they) + would+动词原形
例如：
If I were to see him, I would speak to him about it.
如果我见到他，我会跟他谈这个问题。
（三）错综时间条件句
在错综时间条件句中，虚拟条件从句和主句动作发生的时间不一致，因此，主句和从句的谓语动词要根据各自所指的不同时间选用适当的虚拟语式形式。例如：
Had I taken my umbrella with me when I came out this morning, I should not be wet now.
如果我早上出门时带了伞，现在就不会淋湿了。
If you had spoken to him last time you saw him, you would know what to do now.
如果你上次看到他时跟他说一下，现在就知道怎么做了。
If I were you, I wouldn't have missed the film last night.
如果我是你，我就不会错过昨晚那部电影。
If the ship had left at noon, it would be passing through the canal now.
船要是在中午启航的话，此刻该在通过隧道了。
A wise man wouldn't have done that.
三、无“条件”的虚拟语式句子
这里的所谓无“条件”，指的是句子表层没有通常使用的if或I wish等引起的表示条件的句子，但其深层结构或是上下文中还是有条件的。这种条件可以用介词、形容词、动词、分词、不定式、定语从句或上下文等表示出来。表示无“条件”的词有with, without, but for, or, but, otherwise, even, in case of, what if等。这种句子往往是有主句而无从句。
例如：
But for your help we couldn't have succeeded.
They wouldn't have reached the agreement so easily without that common ground.
上面两句中的bur for 和without 可以用If it were not for来代替。
Under such circumstances I would probably have done the same.
She wasn't feeling very well. Otherwise she wouldn't have left the meeting so early.
I would do so in your place.
With better equipment, we could have done it better.（介词短语）
She said she would have come to the party, only she had an urgent matter to attend to.（副词＋句子）
To talk with him, you would know he is a frank man.（不定式短语）
To have studied harder, you would have passed the exam. (= If you had ..，不定式完成式)
The same thing, happening in war time, would amount to disaster.（分词短语）
On being shown this book, no one would believe that it was written by a ten-year girl.（介词+动名词）
In doing so, he might forgive you. （介词＋动名词）
A man who stopped working would amount to nothing.（定语从句）
He would be a foolish man who should venture that.（定语从句）
You should never join th club, it would do you no good.（上下文）
Give him an inch, he would want a foot.（上下文）
It would be wrong not to take this matter into consideration. (=If you did not take…)
They would have paid attention to a nice girl. (= If she had been…)
Such mistakes could have been avoided. (= If one had been more careful)
She would have acted as you did. (=If she were you)
四、but+表示条件的结构
Would+动词原形+ but (except)＋现在时陈述句（陈述语式）→ 表示同现在事实相反的假设。
Would have +过去分词＋but (except)＋过去时陈述句（陈述语式）→表示同过去事实相反的假设。
例如：
But for her guidance, we should fail.
I would have gone with her but I was too busy.
A:“Why didn't Henry come?”
B:“He would have come except she didn't have time.”
She would gain weight but she doesn't eat enough.
Note:
比较下面两句：
She would have come to see you but she was ill that day.
She would have come to see you but that she was ill that day.
第一句中的but则并列连词，引导的是并列句，but意为“但是”；第二句中的but是介词，意为“除去”。引导条件状语从句。but既可以表示虚拟条件，也可以表示真实条件，但but that只能表示虚拟条件。例如 ：
It never rains but it pours.
She would have bought the book but that she had no money then.
but that结构可以放在句首，并可换为but for+名词结构。例如：
But that I saw it, I couldn't have believed it.
But that it snowed heavily, they could have arrived there earlier. (= But for the heavy snow)
五、虚拟语式在某些从句中的运用
（一）动词wish后的宾语从句用虚拟语式
动词wish后的宾语从句用虚拟语式，与现在事实相反，用were或动词的一般过去式；与过去事实相反，用had+过去分词或would /could+have +过去分词；表示将来没有把握或不太可能实现的愿望，用would / should (could, might)＋动词原形。例如：
I wish she were here.
I wish she had taken my advice.
I wish you would go with us tomorrow.
A:“ You have made some mistakes.”
B:“ I wish I didn't make mistakes every day.”
A:“What would you wish to do if you were a college student again?”
B:“ That's very hard to say, but I wish I had studied psychology.”
Note:
比较下面几个表示未实现的愿望的句子：
I should like to have taken his advice.
我现在真希望（那时）采纳了他的建议。
I should have liked to take his advice.
我本希望采纳他的建议的。
I should have liked to have taken his advice.
（意同上面第二句，但结构笨重）
（二）if only 和would that 后用虚拟语式
if only 和would that 后用虚拟语式，谓语动词用一般过去时表示现在没有实现的愿望，用过去完成时表示过去没有实现的愿望，常译为“要是…就好了”。例如：
If only she had known where to find you.
If only I could speak several foreign languages!
Would that I were young again.
would that 实际上是I (we) would的简化，I would相当于I wish.例如：
I would I were a bird and could fly freely in the sky.
We would that we had seen through him earlier.
Note:
O that 和 Would to God是would that的变体。例如：
O that money grew on the trees.钱长在树上就好了。
Would to God she would return safely.但愿她平安归来！
（三）在suggest, order, demand等后要用虚拟语式
在propose, command, request, desire, insist, require, decide, promise, arrange, ask, move, consent, determine, recommend, advise, intend, beg, resolve, argue, maintain, deserve, prefer, decree, vote, urge, instruct, advocate, persuade, dictate, legislate, sentence, object, expect, direct, appoint, provide, specify, persist, permit等意念动词、祈使动词或态度动词后的宾语从句中，要用虚拟语式，这时，谓语动词用动词原形或should+动词原形。例如：
He determined that she (would) go at once.
They intended that the news (should) be suppressed.
I prefer that you not eat the meat.
She insisted that the seats (should) be booked in advance.
The workers demanded that their wages (should) be raised by 10 percent.
They requested that the (should) sing a song.
He advised that the doctor (should) be sent for.
The committee decided that no one should be admitted without a ticket.
Sometimes she would suggest that I should be saving some of the money.
The examination instructions ask that the students (should) not use a red pen.
It was arranged that she leave the next week.
Note:
When he was asked why his friend had not accepted the offer, he suggested that his friend's pride prevented him from accepting.
句中suggested不作“建议”解，而是作“暗示”解，故用陈述语式。如果上述动词作其他意思解释，或者说话者认为所讲的事是事实，要用陈述语式。比如，当suggest作“暗示、表明”解， insist不表示“坚决要求”，而作“坚决认为”解，其后的从句要用陈述语式，不用虚拟语式。再如：
He insisted that he was not involved in the case.
When asked why he declined our invitation, he suggested that he was not on good terms with the director.
（四）在advice, demand, order等名词后要用虚拟语式
在advice, demand, order等名词后的主语从句和表语从句中用虚拟语式。常用的这类名词有：advice, demand, order, necessity, resolution, pay, decision, proposal, requirement, suggestion, idea, recommendation, preference, request, plan, motion等。这时，谓语用动词原形或should+动词原形。例如：
The demand is that manuscripts (should) be written on one side only.
He issued the order that the troops (should) withdraw at once.
（五）在important, necessary等后用虚拟语式
形容词important, necessary, imperative, natural, urgent, essential, appropriate, desirable, insistent, obligatory, probable, vital, advisable, better, keen, preferable, possible, incredible, funny, concerned, ridiculous, unthinkable, 以及短语no wonder, a pity, not right等可以构成“it is +形容词（名词）+that”句型，其后的句子要用虚拟语式，为(should) be型。例如：
It is necessary that some immediate effort (should) be made.
必须立即采取行动。
It is essential that the program (should) be loaded into computer.
把程序输进计算机非常必要。
Note:
It is amazing (odd, strange, surprising, wrong, astonishing, inconceivable, a pity, a shame, a thousand pities, to be regretted)、 I am surprised (sorry) 和I regret等结构后的that 从句中要用should，意为“竟然”，表示说话人的惊异、懊悔、失望等情感。例如：
It is amazing that he should have learnt so much in such a short time.
I am surprised that he should have been so rude to you.
I regret that I should have forgotten to tell you the fact.
It is a pity that Henry should be so careless.
It is a shame that he should have done such a thing.
有时候，可以将表示情感等的句子前一部分省略，只保留that 以后的部分，成为虚拟式的感叹句。例如：
That he should miss such a golden opportunity! (= It is a pity that…)
That they should resort to violence! (= I am surprised that…)
（六）It is (high, about) time…句型用虚拟语式
It is (high, about) time…句型要求用虚拟语式，谓语动词用过去式，指现在或将来的情况，表示“早该干某事而已经有些晚了”。例如：
It is time that we went to bed.
It is time that somebody taught you to behave yourself.
这种结构中的过去式也可改为should+ 动词原形（用should时，不能将其省略），但不如用过去式普通。例如：
It is about time we should go to bed.
It is time she should wash her clothes.
I suppose it's almost time we were leaving. (should be leaving)
上述两种形式均可用It is time for sb. to do sth.代替。例如：
It is about time for us to go to bed.
It is time for her to wash her clothes.
Note:
下面一句通常只用was,不用were:
It's time I was in bed.
另外，这结构中用about或high是为了缓和或加强语气。
（七）在for fear that, in order that, so that, lest 引导的目的状语从句中
在for fear that, in order that, so that, lest 引导的目的状语从句中，从句谓语动词用should(could, might)＋ 动词原形。在以let 引导的从句中，谓语动词用should+动词原形。例如：
He reminded her twice of it lest she should forget.
I will not make a noise for fear (that ) I should (might) disturb you.
I did this so that I might have a couple of weeks to prepare my paper.
Note:
若in order that 和so that 前的主句是现在时，其后的从句有时亦可用can, may +动词原形。例如：
I didn't mean that you should come so early.
He began to think that we shouldn't have waited so long.
They promised that we should have a good rest after the work.
（八）marvel等动词后，从句用should＋动词结构
marvel, rejoice, wonder, regret等表示强烈感情变化的意念动词，用于否定句、感叹句和疑问句中的think, suppose以及hate可以表示意外地可喜、可惊、可悲、可疑等情况，其宾语从句用should＋动词结构，一般不用be型结构。例如：
I marvel that he should be able to do so much in such a short time.
I regret that you should consider it a burden to do it.
Do you think it proper that she should say that ?
I never thought Helen should be so vain and selfish.
比较:
I hate (it) that you have to endure it.（事实）
I hate that you should consider it a burden to do it.
We rejoice (to learn) that you have come back.
I wonder that she should have given up the job.
（九）imagine等动词后用虚拟句式
imagine和suppose (supposing)可以表示现在或将来非现实情况的意想、态度（用were型结构），也可以表示对过去非实际情况的意想（用had＋过去分词结构）。例如：
Imagine that we were on an island now.
Suppose you were laughed at by those shallow people.
（十）以as if (as though)引导的方式状语从句或表语从句，用虚拟语式
以as if (as though)引导的方式状语从句或表语从句，有时用虚拟语式，从句表示与现在事实相反，谓语动词用一般过去时；表示与过去事实相反，用had＋过去分词；表示将来的可能性不大，用would (might, could) ＋动词原形。例如：
He talks as if he knew all about it.
She looks as though she were sick.
He talks about pyramids as though he had seen them himself.
I feel (felt) as if we had known each other for years.
It looks as if it might rain.
但as if (as though)后面的从句有时也可以用陈述语式，这是因为从句中的情况往往是可能发生的或可能被设想为真实的。例如：
It looks as if our side is going to win.
比较：
He walks as if he is drunk. (= He is probably drunk.)
He walks as if he were drunk. (= He walks as if he were drunk, but he is not.)
Note:
注意 "It isn't as if+虚拟式句子"的译法：
It isn't as if he were poor.
直译：他不像穷的样子。
意译：他才不穷呢。（或：他又不穷！）
Why id you invite him to the symposium? It was not as if he were an expert?
你为什么邀请他参加研讨会？他又不是什么专家！
It isn't as if he had claimed to be the owner of the property. In fact he admitted that the property belonged to his uncle.
他又没有声称自己拥有那些财产呀！事实上，他承认那些财产属于他叔叔。
（十一）在though, whatever 等连接代词或副词引导的状语从句中
在though, whatever, if, so long as, whoever, however, whichever, no matter what (who, how)等连接代词或副词引导的状语从句中，用虚拟语式。这时，谓语动词用动词原形，表示推测，让步等含义，但主句一般用直陈语气。例如：
Though everyone desert you, I will not.
Whatever be his defense, we cannot tolerate this disloyalty.
If the rumour be true, we'll have to take other measures.
So long as a volume hold together, I am not disturbed as to its outer appearance.
Whether he be right or wrong, he'll always go his own way.
Be it true or not, I will see myself.
Happen what may, we will not give in.
Come what may, we're not going to make any concessions to his unreasonable demands.
Be the weather what it may, we will leave tomorrow.
Note:
when, granted that等引导的让步状语从句有时亦用虚拟语式。例如：
She walks when he might take a taxi. (=though)
Granted that you had been drunk, there is no excuse for you to do that.
（十二）expect等动词的否定式或疑问式后
expect, believe, suspect, dream等动词用于否定句或疑问句时，其后的宾语从句一般用should ＋ 动词，表示惊讶、惶恐、怀疑、失望、不满等情绪。注意，这种结构中的should不可省略。例如：
He just couldn't believe that his hometown should have gone through such great changes.
（十三）would rather, would sooner,等后的句子
would rather, would sooner, had better, would as soon, just as soon后的句子用虚拟语式。
A)接动词原形，指现在或将来时间，表示一种主观愿望或选择。例如：
I'd rather do it today.
He would sooner die than surrender.
He'd rather not go dancing tonight.
B)接不定式完成式，表示过去的某种选择不恰当。例如：
I'd rather not have told her the news. She is such a gossip.
I'd rather have stayed at home than went to the dull film.
C)接从句，用虚拟语式，为一般过去时，表示一个现在或将来的愿望；用过去完成时，表示一个过去的愿望。例如：
I'd rather you paid the money yourself。（现在）
I would sooner she painted the wall green next time.（将来）
I'd rather she hadn't done that.（过去）
I'd soon you didn't do anything about it for the time being.
（十四）would rather …than 和would…rather than后
would rather …than 和would…rather than后的句子用虚拟语式，这两个结构中than后的从句要用should＋动词原形。例如：
I would rather die than (that) he should know the secret.
He'd do anything rather than (that) he should live with such a shameless woman.
（十五）would, should, could和might用于虚拟语式中
would, should, might和could均可用于虚拟语式中，但在含义和用法上有些差别：
A)第一人称可用should或would，其他人称多用would。
B)would 可以表示情态意义，表示主语的意想和意图，通用于所有人称，should表示意愿和意见时，只用于第二、第三人称。
C)could可以表示“能力”，指由知识、技能、体力等所产生的能力。
D)could可以表示“允许，许诺”，相当于would be allowed to, 但might则无此义。
E)表示“可能”时，could和might可以互换，但在否定句中多用could。
F) ought to表示推断和理应如何，有时亦用must对过去表示推断。例如：
I would lend him the money if he asked me.（意愿）
If he had time, he should do it.（意愿）
If he forgot to come, you could go instead.（许诺）
If she started early, she ought to be here by now.（推断）
If you had been here this morning, you must have seen the famous writer.（推断）
六、条件句及虚拟式的其他表示法
（一）so (as) long as
so (as) long as 表示一个真实条件，谓语动词用陈述语式。
例如：
You may borrow the book so long as you return it in one week.
（二）on condition that
on condition that 意为“在…条件下”，它所引导的从句谓语动词可以用陈述语式，也可以用虚拟语式，但均表示真实条件。
例如：
The guards allow the tourists to go into the woods on condition that they do (should do) no harm to the animals
参考资料： 够详细吧，呵呵
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虚拟语气(Subjunctive Mood)这一语法项目是各类英语考试中心测试的重点之一。虚拟语气是一种特殊的动词形式，用来表示说话人所说的话并不是事实，而是一种假设、愿望、怀疑或推测。
Ⅰ用以表示虚拟条件的虚拟语气
⒈ 用if条件从句表示的虚拟条件，是虚拟条件最普通的方式。
① 虚拟现在时表示与现在事实相反的假设，其if 从句的谓语形式用动词的过去式(be 一般用were)，主句用would/ should/ could/ might +动词原形，例如：If I were in your position I would marry her.
② 虚拟过去时是表示与过去事实相反的假设，if 从句的谓语形式用过去完成时即had＋过去分词，主句用would / should / could / might + have +过去分词，例如：If it had not rained so hard yesterday we could have played tennis.
③ 大多数的虚拟条件句属于上面三种情况的一种，但并不排除存在条件和后果中，一个和现在情况相反，另一个和过去情况相反，例如：If you had followed what the doctor said, you would not have been so painful now.这个句子在高中出现频率颇高。
④ 但是，如果后果用了虚拟语气，而条件却用陈述语气，这种用法是错的。
⒉ 除了表示虚假条件外，if从句还可以表示对将来的推测，由于是将来还没有发生的，所以谈不上是真实的还是虚假的，只能说这个事情发生的可能性有多大。一般情况下，可以用陈述语气的if从句来表示对一个未来事实的推测，这个事实是完全可能发生的。If从句的谓语形式用一般过去式或用were to / should +动词原形，主句用would / should /could/ might +动词原形，例如：Jean doesn't want to work right away because she thinks that if she were to get a job she probably wouldn't be able to see her friends very often.（1996年1月四级第44题）
⒊ 有时可以把含有助动词、情态动词、be或have的虚拟条件句中的连词if 省去，而将had , should, were 等词提到主语之前，即用倒装结构,这时候，如果出现not等否定词，否定词需放在主语后面。这种结构在口语中很少使用，但频频出现在各类考试中出现，例如：If it had not rained so hard yesterday we could have played tennis.→Had it not rained so hard yesterday, we could have played tennis.
⒋ 大多数的虚拟条件通过上面所讲的两种方法表达，但在个别句子中也可以通过介词without和介词短语but for表达，副词otherwise等表达出来。例如：We didn't know his telephone number; otherwise we would have telephoned him.（1995年6月四级第46题）值得注意的是，包含but for的句子，谓语动词必定要用虚拟形式，但包含without等短语的句子，谓语动词未必一定要用虚拟语气。
⒌ 有时虚拟条件不是明确地表达出来，而在蕴含在用but引导的从句里，于是便出现了有谓语动词是虚拟语气的主句加上谓语动词是陈述语气的but从句构成的并列复合句，例如I would have hung you but the telephone was out of order.在这样的句子里，如果主句没有用虚拟语气，或者从句用了虚拟语气，都是错的。
Ⅱ用在宾语从句中的虚拟语气
⒈ 在表示愿望的动词wish后的宾语从句中，需用虚拟语气。（wish后的that 常省略），根据主句时态，从句谓语时态相应退后一位，例如：How she wished his family could go with him.
⒉ 在具有愿望、请求、建议、命令等主观意愿的动词(desire, demand, advice, insist, require, suggest, propose, order, recommend, decide …) 后的宾语从句中需用虚拟语气。谓语动词用(should) +动词原形。值得注意的是，如果宾语从句的动词是否定的，否定词not的位置应在动词之前，而不是动词之后。例如：The head nurse insisted that the patient not be move.另外，如suggest表达“暗示”，insist表示“坚持某种说法”时，后面的从句不用虚拟语气，例如：Her pale face suggests that she is ill.或He insisted that he did not kill the boy.
除此之外，上述动词也要求用虚拟式
① 在It is+上述动词的过去分词，其后所跟的主语从句中，如It is suggested that pupils wear school uniforms.
② 在上述动词相应的名词形式作主语+连系动词，其后的表语从句中，例如His suggestion was that classed be re-scheduled.
③ 在对上述动词相应的名词进行解释的同位语从句中，如：The workers raised the demand that their pay be increased to cope with the inflation.
⒊ would （had）rather , would sooner也用来表达主观愿望，它们之后的宾语从句中需用虚拟语气。谓语动词用过去式表示现在或将来，用过去完成式表示与过去事实相反，例如：I would rather he went right now.
Ⅲ其他形式的虚拟语气
⒈ it is +necessary等形容词后，that主语从句中虚拟形式使用，这类形容词包括necessary, important, essential, imperative, urgent, preferable, vital, advisable等，例如It is necessary that you listen to the teacher carefully.或者It is essential that you be able to pronounce every single word correctly.
⒉ 在It's (high/about) time 之后的定语从句中需用虚拟语气。谓语动词用过去式，例如It is high time that you went to school
⒊ 虚拟语气在as if / as though 引导的方式状语从句中的应用（谓语动词形式与wish后的宾语从句基本相同）表示与现在事实相反或对现在情况有怀疑，谓语动词用过去式。例如：He felt as if he alone were responsible for what had happened. 表示过去想象中的动作或情况，谓语动词用过去完成式。
⒋ 在lest 引导的状语从句中，谓语动词多用虚拟语气，(should )+ 动词原形。例如： The mad man was put in the soft-padded cell lest he injure himself.（1998年1月四级第38题）
⒌ 在if only 引起的感叹句中需用虚拟语气。谓语动词用过去式或过去完成式。例如： Look at the terrible situation I am in! if only I had followed your advice.
Ⅰ用以表示虚拟条件的虚拟语气
⒈ 用if条件从句表示的虚拟条件，是虚拟条件最普通的方式。
① 虚拟现在时表示与现在事实相反的假设，其if 从句的谓语形式用动词的过去式(be 一般用were)，主句用would/ should/ could/ might +动词原形，例如：If I were in your position I would marry her.
② 虚拟过去时是表示与过去事实相反的假设，if 从句的谓语形式用过去完成时即had＋过去分词，主句用would / should / could / might + have +过去分词，例如：If it had not rained so hard yesterday we could have played tennis.
③ 大多数的虚拟条件句属于上面三种情况的一种，但并不排除存在条件和后果中，一个和现在情况相反，另一个和过去情况相反，例如：If you had followed what the doctor said, you would not have been so painful now.这个句子在高中出现频率颇高。
④ 但是，如果后果用了虚拟语气，而条件却用陈述语气，这种用法是错的。
⒉ 除了表示虚假条件外，if从句还可以表示对将来的推测，由于是将来还没有发生的，所以谈不上是真实的还是虚假的，只能说这个事情发生的可能性有多大。一般情况下，可以用陈述语气的if从句来表示对一个未来事实的推测，这个事实是完全可能发生的。If从句的谓语形式用一般过去式或用were to / should +动词原形，主句用would / should /could/ might +动词原形，例如：Jean doesn't want to work right away because she thinks that if she were to get a job she probably wouldn't be able to see her friends very often.（1996年1月四级第44题）
⒊ 有时可以把含有助动词、情态动词、be或have的虚拟条件句中的连词if 省去，而将had , should, were 等词提到主语之前，即用倒装结构,这时候，如果出现not等否定词，否定词需放在主语后面。这种结构在口语中很少使用，但频频出现在各类考试中出现，例如：If it had not rained so hard yesterday we could have played tennis.→Had it not rained so hard yesterday, we could have played tennis.
⒋ 大多数的虚拟条件通过上面所讲的两种方法表达，但在个别句子中也可以通过介词without和介词短语but for表达，副词otherwise等表达出来。例如：We didn't know his telephone number; otherwise we would have telephoned him.（1995年6月四级第46题）值得注意的是，包含but for的句子，谓语动词必定要用虚拟形式，但包含without等短语的句子，谓语动词未必一定要用虚拟语气。
⒌ 有时虚拟条件不是明确地表达出来，而在蕴含在用but引导的从句里，于是便出现了有谓语动词是虚拟语气的主句加上谓语动词是陈述语气的but从句构成的并列复合句，例如I would have hung you but the telephone was out of order.在这样的句子里，如果主句没有用虚拟语气，或者从句用了虚拟语气，都是错的。
Ⅱ用在宾语从句中的虚拟语气
⒈ 在表示愿望的动词wish后的宾语从句中，需用虚拟语气。（wish后的that 常省略），根据主句时态，从句谓语时态相应退后一位，例如：How she wished his family could go with him.
⒉ 在具有愿望、请求、建议、命令等主观意愿的动词(desire, demand, advice, insist, require, suggest, propose, order, recommend, decide …) 后的宾语从句中需用虚拟语气。谓语动词用(should) +动词原形。值得注意的是，如果宾语从句的动词是否定的，否定词not的位置应在动词之前，而不是动词之后。例如：The head nurse insisted that the patient not be move.另外，如suggest表达“暗示”，insist表示“坚持某种说法”时，后面的从句不用虚拟语气，例如：Her pale face suggests that she is ill.或He insisted that he did not kill the boy.
除此之外，上述动词也要求用虚拟式
① 在It is+上述动词的过去分词，其后所跟的主语从句中，如It is suggested that pupils wear school uniforms.
② 在上述动词相应的名词形式作主语+连系动词，其后的表语从句中，例如His suggestion was that classed be re-scheduled.
③ 在对上述动词相应的名词进行解释的同位语从句中，如：The workers raised the demand that their pay be increased to cope with the inflation.
⒊ would （had）rather , would sooner也用来表达主观愿望，它们之后的宾语从句中需用虚拟语气。谓语动词用过去式表示现在或将来，用过去完成式表示与过去事实相反，例如：I would rather he went right now.
Ⅲ其他形式的虚拟语气
⒈ it is +necessary等形容词后，that主语从句中虚拟形式使用，这类形容词包括necessary, important, essential, imperative, urgent, preferable, vital, advisable等，例如It is necessary that you listen to the teacher carefully.或者It is essential that you be able to pronounce every single word correctly.
⒉ 在It's (high/about) time 之后的定语从句中需用虚拟语气。谓语动词用过去式，例如It is high time that you went to school
⒊ 虚拟语气在as if / as though 引导的方式状语从句中的应用（谓语动词形式与wish后的宾语从句基本相同）表示与现在事实相反或对现在情况有怀疑，谓语动词用过去式。例如：He felt as if he alone were responsible for what had happened. 表示过去想象中的动作或情况，谓语动词用过去完成式。
⒋ 在lest 引导的状语从句中，谓语动词多用虚拟语气，(should )+ 动词原形。例如： The mad man was put in the soft-padded cell lest he injure himself.（1998年1月四级第38题）
⒌ 在if only 引起的感叹句中需用虚拟语气。谓语动词用过去式或过去完成式。例如： Look at the terrible situation I am in! if only I had followed your advice.
参考资料： http://zhidao.baidu.com/question/1847556.html
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虚拟语气
虚拟语气(Subjunctive Mood)这一语法项目是各类英语考试中心测试的重点之一。虚拟语气是一种特殊的动词形式，用来表示说话人所说的话并不是事实，而是一种假设、愿望、怀疑或推测。
Ⅰ用以表示虚拟条件的虚拟语气
⒈ 用if条件从句表示的虚拟条件，是虚拟条件最普通的方式。
① 虚拟现在时表示与现在事实相反的假设，其if 从句的谓语形式用动词的过去式(be 一般用were)，主句用would/ should/ could/ might +动词原形，例如：If I were in your position I would marry her.
② 虚拟过去时是表示与过去事实相反的假设，if 从句的谓语形式用过去完成时即had＋过去分词，主句用would / should / could / might + have +过去分词，例如：If it had not rained so hard yesterday we could have played tennis.
③ 大多数的虚拟条件句属于上面三种情况的一种，但并不排除存在条件和后果中，一个和现在情况相反，另一个和过去情况相反，例如：If you had followed what the doctor said, you would not have been so painful now.这个句子在高中出现频率颇高。
④ 但是，如果后果用了虚拟语气，而条件却用陈述语气，这种用法是错的。
⒉ 除了表示虚假条件外，if从句还可以表示对将来的推测，由于是将来还没有发生的，所以谈不上是真实的还是虚假的，只能说这个事情发生的可能性有多大。一般情况下，可以用陈述语气的if从句来表示对一个未来事实的推测，这个事实是完全可能发生的。If从句的谓语形式用一般过去式或用were to / should +动词原形，主句用would / should /could/ might +动词原形，例如：Jean doesn't want to work right away because she thinks that if she were to get a job she probably wouldn't be able to see her friends very often.（1996年1月四级第44题）
⒊ 有时可以把含有助动词、情态动词、be或have的虚拟条件句中的连词if 省去，而将had , should, were 等词提到主语之前，即用倒装结构,这时候，如果出现not等否定词，否定词需放在主语后面。这种结构在口语中很少使用，但频频出现在各类考试中出现，例如：If it had not rained so hard yesterday we could have played tennis.→Had it not rained so hard yesterday, we could have played tennis.
⒋ 大多数的虚拟条件通过上面所讲的两种方法表达，但在个别句子中也可以通过介词without和介词短语but for表达，副词otherwise等表达出来。例如：We didn't know his telephone number; otherwise we would have telephoned him.（1995年6月四级第46题）值得注意的是，包含but for的句子，谓语动词必定要用虚拟形式，但包含without等短语的句子，谓语动词未必一定要用虚拟语气。
⒌ 有时虚拟条件不是明确地表达出来，而在蕴含在用but引导的从句里，于是便出现了有谓语动词是虚拟语气的主句加上谓语动词是陈述语气的but从句构成的并列复合句，例如I would have hung you but the telephone was out of order.在这样的句子里，如果主句没有用虚拟语气，或者从句用了虚拟语气，都是错的。
Ⅱ用在宾语从句中的虚拟语气
⒈ 在表示愿望的动词wish后的宾语从句中，需用虚拟语气。（wish后的that 常省略），根据主句时态，从句谓语时态相应退后一位，例如：How she wished his family could go with him.
⒉ 在具有愿望、请求、建议、命令等主观意愿的动词(desire, demand, advice, insist, require, suggest, propose, order, recommend, decide …) 后的宾语从句中需用虚拟语气。谓语动词用(should) +动词原形。值得注意的是，如果宾语从句的动词是否定的，否定词not的位置应在动词之前，而不是动词之后。例如：The head nurse insisted that the patient not be move.另外，如suggest表达“暗示”，insist表示“坚持某种说法”时，后面的从句不用虚拟语气，例如：Her pale face suggests that she is ill.或He insisted that he did not kill the boy.
除此之外，上述动词也要求用虚拟式
① 在It is+上述动词的过去分词，其后所跟的主语从句中，如It is suggested that pupils wear school uniforms.
② 在上述动词相应的名词形式作主语+连系动词，其后的表语从句中，例如His suggestion was that classed be re-scheduled.
③ 在对上述动词相应的名词进行解释的同位语从句中，如：The workers raised the demand that their pay be increased to cope with the inflation.
⒊ would （had）rather , would sooner也用来表达主观愿望，它们之后的宾语从句中需用虚拟语气。谓语动词用过去式表示现在或将来，用过去完成式表示与过去事实相反，例如：I would rather he went right now.
Ⅲ其他形式的虚拟语气
⒈ it is +necessary等形容词后，that主语从句中虚拟形式使用，这类形容词包括necessary, important, essential, imperative, urgent, preferable, vital, advisable等，例如It is necessary that you listen to the teacher carefully.或者It is essential that you be able to pronounce every single word correctly.
⒉ 在It's (high/about) time 之后的定语从句中需用虚拟语气。谓语动词用过去式，例如It is high time that you went to school
⒊ 虚拟语气在as if / as though 引导的方式状语从句中的应用（谓语动词形式与wish后的宾语从句基本相同）表示与现在事实相反或对现在情况有怀疑，谓语动词用过去式。例如：He felt as if he alone were responsible for what had happened. 表示过去想象中的动作或情况，谓语动词用过去完成式。
⒋ 在lest 引导的状语从句中，谓语动词多用虚拟语气，(should )+ 动词原形。例如： The mad man was put in the soft-padded cell lest he injure himself.（1998年1月四级第38题）
⒌ 在if only 引起的感叹句中需用虚拟语气。谓语动词用过去式或过去完成式。例如： Look at the terrible situation I am in! if only I had followed your advice.
虚拟语气(Subjunctive Mood)这一语法项目是各类英语考试中心测试的重点之一。虚拟语气是一种特殊的动词形式，用来表示说话人所说的话并不是事实，而是一种假设、愿望、怀疑或推测。
Ⅰ用以表示虚拟条件的虚拟语气
⒈ 用if条件从句表示的虚拟条件，是虚拟条件最普通的方式。
① 虚拟现在时表示与现在事实相反的假设，其if 从句的谓语形式用动词的过去式(be 一般用were)，主句用would/ should/ could/ might +动词原形，例如：If I were in your position I would marry her.
② 虚拟过去时是表示与过去事实相反的假设，if 从句的谓语形式用过去完成时即had＋过去分词，主句用would / should / could / might + have +过去分词，例如：If it had not rained so hard yesterday we could have played tennis.
③ 大多数的虚拟条件句属于上面三种情况的一种，但并不排除存在条件和后果中，一个和现在情况相反，另一个和过去情况相反，例如：If you had followed what the doctor said, you would not have been so painful now.这个句子在高中出现频率颇高。
④ 但是，如果后果用了虚拟语气，而条件却用陈述语气，这种用法是错的。
⒉ 除了表示虚假条件外，if从句还可以表示对将来的推测，由于是将来还没有发生的，所以谈不上是真实的还是虚假的，只能说这个事情发生的可能性有多大。一般情况下，可以用陈述语气的if从句来表示对一个未来事实的推测，这个事实是完全可能发生的。If从句的谓语形式用一般过去式或用were to / should +动词原形，主句用would / should /could/ might +动词原形，例如：Jean doesn't want to work right away because she thinks that if she were to get a job she probably wouldn't be able to see her friends very often.（1996年1月四级第44题）
⒊ 有时可以把含有助动词、情态动词、be或have的虚拟条件句中的连词if 省去，而将had , should, were 等词提到主语之前，即用倒装结构,这时候，如果出现not等否定词，否定词需放在主语后面。这种结构在口语中很少使用，但频频出现在各类考试中出现，例如：If it had not rained so hard yesterday we could have played tennis.→Had it not rained so hard yesterday, we could have played tennis.
⒋ 大多数的虚拟条件通过上面所讲的两种方法表达，但在个别句子中也可以通过介词without和介词短语but for表达，副词otherwise等表达出来。例如：We didn't know his telephone number; otherwise we would have telephoned him.（1995年6月四级第46题）值得注意的是，包含but for的句子，谓语动词必定要用虚拟形式，但包含without等短语的句子，谓语动词未必一定要用虚拟语气。
⒌ 有时虚拟条件不是明确地表达出来，而在蕴含在用but引导的从句里，于是便出现了有谓语动词是虚拟语气的主句加上谓语动词是陈述语气的but从句构成的并列复合句，例如I would have hung you but the telephone was out of order.在这样的句子里，如果主句没有用虚拟语气，或者从句用了虚拟语气，都是错的。
Ⅱ用在宾语从句中的虚拟语气
⒈ 在表示愿望的动词wish后的宾语从句中，需用虚拟语气。（wish后的that 常省略），根据主句时态，从句谓语时态相应退后一位，例如：How she wished his family could go with him.
⒉ 在具有愿望、请求、建议、命令等主观意愿的动词(desire, demand, advice, insist, require, suggest, propose, order, recommend, decide …) 后的宾语从句中需用虚拟语气。谓语动词用(should) +动词原形。值得注意的是，如果宾语从句的动词是否定的，否定词not的位置应在动词之前，而不是动词之后。例如：The head nurse insisted that the patient not be move.另外，如suggest表达“暗示”，insist表示“坚持某种说法”时，后面的从句不用虚拟语气，例如：Her pale face suggests that she is ill.或He insisted that he did not kill the boy.
除此之外，上述动词也要求用虚拟式
① 在It is+上述动词的过去分词，其后所跟的主语从句中，如It is suggested that pupils wear school uniforms.
② 在上述动词相应的名词形式作主语+连系动词，其后的表语从句中，例如His suggestion was that classed be re-scheduled.
③ 在对上述动词相应的名词进行解释的同位语从句中，如：The workers raised the demand that their pay be increased to cope with the inflation.
⒊ would （had）rather , would sooner也用来表达主观愿望，它们之后的宾语从句中需用虚拟语气。谓语动词用过去式表示现在或将来，用过去完成式表示与过去事实相反，例如：I would rather he went right now.
Ⅲ其他形式的虚拟语气
⒈ it is +necessary等形容词后，that主语从句中虚拟形式使用，这类形容词包括necessary, important, essential, imperative, urgent, preferable, vital, advisable等，例如It is necessary that you listen to the teacher carefully.或者It is essential that you be able to pronounce every single word correctly.
⒉ 在It's (high/about) time 之后的定语从句中需用虚拟语气。谓语动词用过去式，例如It is high time that you went to school
⒊ 虚拟语气在as if / as though 引导的方式状语从句中的应用（谓语动词形式与wish后的宾语从句基本相同）表示与现在事实相反或对现在情况有怀疑，谓语动词用过去式。例如：He felt as if he alone were responsible for what had happened. 表示过去想象中的动作或情况，谓语动词用过去完成式。
⒋ 在lest 引导的状语从句中，谓语动词多用虚拟语气，(should )+ 动词原形。例如： The mad man was put in the soft-padded cell lest he injure himself.（1998年1月四级第38题）
⒌ 在if only 引起的感叹句中需用虚拟语气。谓语动词用过去式或过去完成式。例如： Look at the terrible situation I am in! if only I had followed your advice.
参考资料： http://zhidao.baidu.com/question/810512.html
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退时态：真实语气中的现在时变成过去时，过去时变成过去完成时，将来时变成过去将来时
1．条件句中：
If it were not for…；If it hadn’t been for…这两种句型表示“要是没有”，相当于but for
If it had not been for the PLA man, he would not be studying English now.
if…only…（或If only…）表示“要是…就好了”
1．名词性从句：
demand/suugest/order/insist/propose/request/ask/advise/prefer/command/desire that…
They demanded that the aggressor troops (should) be withdrawn immediately.
由insist引导的宾语从句可以虚拟也可以不虚拟：
He insisted that he was right.坚持认为
He insisted that we should agree with him.坚持某事
由suggest引导的宾语从句可以虚拟也可以不虚拟：
His white face suggests that his not healthy.表明
He suggested that we have a rest.建议
If I could only see him once!
If only I had listened to your advice. 很明显，这种句型省略了主句
2．其他状语从句中：
as if/as though/even if…
Tom looked sad as if he had heard bad news.
I won’t lend you any money even I had money.
3．名词性从句：
wish that…（hope后的宾语从句不虚拟）
I wish I were as strong as you.
How I wish it wasn’t raining!
would rather/would sooner/had rather that…表示“宁愿”
I would rather they came tomorrow.
I could go myself but I would sooner you went.
I had rather you told him than I did.
It is time/high time that…
It is time we went to bed.
+(should) do：
1．条件句中：
If it were not for…；If it hadn’t been for…这两种句型表示“要是没有”，相当于but for
If it had not been for the PLA man, he would not be studying English now.
if…only…（或If only…）表示“要是…就好了”
1．名词性从句：
demand/suugest/order/insist/propose/request/ask/advise/prefer/command/desire that…
They demanded that the aggressor troops (should) be withdrawn immediately.
由insist引导的宾语从句可以虚拟也可以不虚拟：
He insisted that he was right.坚持认为
He insisted that we should agree with him.坚持某事
由suggest引导的宾语从句可以虚拟也可以不虚拟：
His white face suggests that his not healthy.表明
He suggested that we have a rest.建议
If I could only see him once!
If only I had listened to your advice. 很明显，这种句型省略了主句
2．其他状语从句中：
as if/as though/even if…
Tom looked sad as if he had heard bad news.
I won’t lend you any money even I had money.
3．名词性从句：
wish that…（hope后的宾语从句不虚拟）
I wish I were as strong as you.
How I wish it wasn’t raining!
would rather/would sooner/had rather that…表示“宁愿”
I would rather they came tomorrow.
I could go myself but I would sooner you went.
I had rather you told him than I did.
It is time/high time that…
It is time we went to bed.
+(should) do：
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虚拟语气是动词的一种特殊形式,用来表达说话人的意愿.请求,设想,等未能或不可能实现的事实,或说话人看来实现可能性很小的情况.
下面的网址有具体分析:
http://www.lesun.org/english/html/2005-02/334.htm
http://www.mryuan.net/ShowArticle.asp?ArticleID=55
下面的网址有具体分析:
http://www.lesun.org/english/html/2005-02/334.htm
http://www.mryuan.net/ShowArticle.asp?ArticleID=55
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