英语If用法
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if作为从属连词,可引导条件状语从句、让步状语从句和名词从句。
if引导条件状语从句,从句可为真实条件从句,也可为虚拟条件从句。如为真实条件从句,谓语用陈述语气,表示可能性很大,作“假使”“如果”解,通常用一般现在时代替一般将来时; 如果if从句中用will,表示意愿。从句如为虚拟条件从句,谓语动词要用虚拟语气,表示可能性不大或与过去事实相反,作“要是,假如”解。可指过去的情况,也可指现在和将来的情况。指将来情况时可用were to或should。在现代口语中,一、三人称后常用was代替were。if引导的虚拟条件句中,助动词were,should,had可以移至主语前,此时if可省略。
if引导的条件从句中可以省略be和主语。
if引导的条件从句,有时后接only以加强其语气,表示说话人非常盼望某人提出的条件能够实现。
if可引导让步状语从句,作“虽然,即使”解,常和even连用。
if可引导名词性从句,作“是否”解,常放在ask, doubt, know, learn, see, wonder等动词后。一般用在口语中,而且不能用于句首。
if可以表示惊奇或愤怒。用于陈述语气的否定句时,意义则是肯定的。
If ever...在现代英语中,表示强调,用于加强主句的意思,往往含有否定意思。
When or if〔when, or if〕表示从句所说的也许不会实现。
if any的意思是“如果有的话”。
if only可表示对现时或未来的愿望,也可表示与过去事实相反的愿望,作“只要…但愿,要是…就好了”解。
What will happen if...?意思是“要是…怎么办”。常缩略成What if...?
If not后可加动词,通常动词省略,可用于现在时态或现在完成时态,表示“要不,不然”; 也可用于yes〔no〕疑问句之后,表示“许诺,告诫”。
If you demand his presence,warn him in advance.
如果你要他来,就事先通知他。If you will be so kind as to help us, we can finish by six.
如果你愿意帮助我们,到六点时咱们就能完成了。If you were a bird,you could fly.
如果你是鸟,你就能飞行了。I should have talked to mother if I had thought of it.
如果当初我想到了,我就和妈妈说了。If I were to ask him for a loan,would he agree ?
如果我向他借钱的话,他能同意吗?If it should rain tomorrow,the rally would be postponed.
要是明天下雨,群众大会就会延期举行。If I were an architect,I'd redesign this house.
如果我是建筑师,我会重新设计这房子。If it were up there, it would be easier to find.
如果它在那儿,就会更容易被找到。If had anything happened, he would have let me know.
如果当初发生了什么事,他会让我知道的。If necessary, give me a ring.
如果需要,给我打个电话。He will come, if invited.
如果受到邀请,他会来的。If only somebody had told us, we could have warned you.
如果有人告诉了我们,我们就能事先提醒你。We will go even if it rains.
即使下雨,我们也要去。If he is poor, at least he is honest.
他虽然穷,但至少是诚实的。I don't know if he was there.
我不知道他当初是否在那里。
来时，则if引导的状语从句用一般现在时（最基本的）。
eg.
If
it
doesn’t
rain,
we
will
go
to
the
picnic
next
Sunday.
如果天不下雨，下周日我们将去野餐。二、if还可以引导让步状语从句。在意义上相当于even
if,even
though或although。这时
if
意为“即使
是”，“虽然”。
eg.
I’ll
do
it,
even
if
it
takes
me
a
long
time．
虽然会花费很长时间，我还是要做这事。
If
he is
little,he is
strong.他年纪虽小,力气倒大
三、if作为连词还可以引导宾语从句。引导宾语从句时和whether意思相同。例如：
eg.
She
asked
if
／
whether
I
like
it．
她问我是否喜欢它。
*
此时注意：1.
or
not放在whether之后时，只能用whether不用if。 2.
在强调任意选择时,用whether…or,此时不用if替换whether
3.
虽引导宾语从句，但为了强调宾语部分，也可把从句放在句首，此时只用whether
不用if 。 4.
从句作介词宾语时只用whether不用if。
5.
作discuss等动词的宾语时，用whether不用if。 6. 句子中有if引导的条件句，如再有表示“是否”的宾语从句，用whether不用if
7. 容易产生歧义时用whether不用if来表示“是否”。
基本上碰到选择
if
还是
whether
的问题时都选
whether，除了一条：如果宾语从句为否定句时，则
只用if不用whether。eg.
I
don’t
know
if
it
won't
rain
tomorrow..我不知道明天是不是会下雨
四、if引导时间状语从句，意为“当”或“无论何时”，不含有条件之义时，if从句中的时态与主句中的时 态相同。
eg.
If
you
mix
yellow
and
blue，you
get
green．你将黄色与蓝色混合，便会得到绿色。
五、if后接否定动词，用于感叹句中，表示沮丧、惊奇等。例如：
eg.
Well，if
I
haven’t
left
my
homework
at
home!
真倒霉，我把作业落在家里了!
六、用于虚拟语气中，if从句中用过去式，表示不太可能实现或提出作为考虑的假设条件。eg.
1.
If I
were
a
bird, I
could
fly.如果我是只鸟，我就会飞了。
2.
If
I
asked
him(if
I
were
to
ask
him)
for
a
loan，would
he
agree?如果我向他借款，他会答应吗?
另外，if从句中用过去完成式，表示过去未实现的条件。eg.
1.
If
they
had
started
earlier，they
would
have
arrived
in
time．要是他们早些动身，他们便可及时到达了。
2.
If
they
had
not
started
at
that
time，they
would
not
be
here
now．如果他们那时不动身，现在他们就不会在此地了。
第三，if
条件句的虚拟有时可省略，然后将主语与所用的限定动词(were，should，had)倒置。eg.
1.
Should
it
(=if
it
should)
be
necessary，I
will
go.
倘若有必要，我会去的。
2.
Were
I(=if
I
were)
in
your
post，I
would
do
the
same
．
如果我处于你的位置，我也会这样干的。
3.
Had
I(=if
I
had)known
earlier，I
wouldn't
go．如果我早一点知道，我就会不会走了。
七、if与其它一些介词的连用。
as与if连用，意思是：“仿佛、好像”同样引导从句。eg.
It
isn’t
as
if
he
doesn’t
know
the
rules．(ie．
He
does
know
the
rulers．)他不像不懂规则的样子。
(即他懂得规则)
（ie=namely=that
is
to
say=in
other
words：
“即，那就是说”）
另外as
if还可以引导感叹句。例如：
As
if
I
would
allow
it!
(ie．I
would
certainly
not
allow
it!)
好像我会答应那样!
(我才不会答应呢!)
*
if与only连用。表示“如果...就好了”。例如：
1.
If
only
he
arrives
in
time!他若能及时到达就好了!
2.
If
only
she
had
known
about
it!（But
she
didn’t
know.）她那时要是知道（但她不知道）这事就好了!
*
此时注意区别
only
if...：“只要...”，相当于“if”
eg.
Only
if
you
word
hard，you'll
be
successful
one
day.
He will come if you invite him.
如果你请他,他会来的.
2.(表示虚拟)假如,要是
1）时态：可以表示过去,现在和将来的情况.它的基本特点是时态退后.
a.同现在事实相反的假设.
句型 ：条件从句 主句
一般过去时 should( would) +动词原形
If they were here,they would help you.
b.表示于过去事实相反的假设.
句型：条件从句 主句
过去完成时 should(would) have+ 过去分词
If she had worked harder,she would have succeeded.
The rice would not have been burnt if you had been more careful.
If my lawyer had been here last Saturday,he would have prevented me from going.
If he had come yesterday,I should / would have told him about it.
含义：He did not come yesterday,so I did not tell him about it.
If he had not been ill and missed many classes,he would have made greater progress.
含义：He was ill and missed many lessons,so he did not make greater progress.
c.表示对将来的假想
句型：条件从句 主句
一般过去时 should+ 动词原形
were+ 不定式 would + 动词原形
should+ 动词原形
If you succeeded,everything would be all right.
If you should succeed,everything would be all right.
If you were to succeed,everything would be all right.
3.是否 = whether 连接宾语从句
I wonder if she is ill.
不知她是否病了.
如：We'll go to the supermarket if it doesn't rain
(但请你注意：1.主句可以用祈使句来表示将来时，这时主句用一般现在时 Put on your coat if you feel cold
2.是真理的话，前后都用一般现在时)
解释为是否时，用法同whether
E.g.: I dont't if he has passed exam
也可以用whether ,前后时态统一
用主将从现的还有：as soon as, when, before, until,unless，after……
这类词有很多情况都是用过去时的，但若主句是用将来时，从句就用一般现在时
如：I'll call you as soon as he comes back.
其他的也要注意主句的时态就可以判断了
