Question

定语从句中什么时候用it什么时候用 that

Answer 1

A. 以下情况多用that.
(1) 如果先行词是all, much, anything, something, nothing, everything, little, none等不定代词，关系代词一般只用that，不用which。例如：
All the people that are present burst into tears.
Is there anything that I can do for you?
(2) 如果先行词被the only, the very, the same, the last, the best等词修饰，关系代词常用that(指人时，可以用who),例如：
This is the very book that I want to find.
That is the best film that we have seen.
I was the only person in my office that/who was invited.

(3) 当先行词被all, every, no, some, any, little, much所修饰时；
You can take any seat that is free.
(4) 当先行词被序数词或形容词最高级所修饰时。
It is the first foreign book that I have ever seen.
This is the most beautiful flower that I have seen.
(5) 在there be句型中，常用that.
There is a man that lives in that village.
(6) 先行词有两个，一个指人，一个指物，关系代词应该用that。例如：
The boy and the dog that are in the picture are very lovely.
B. 以下情况用which.
(1) 当关系词前面有介词时，指物常用which.
The prize for which he worked so hard was a new bike.
He built a telescope through which he could study the skies.
(2) 非限制性定语从句指物时，多用which.
Bamboo is hollow, which makes it light.
He wrote an article, which tells us about the computer.
(3) 如果先行词本身是that时，只用which.
This is that which he bought yesterday.
(4) which还有一种特殊用法，它可以引导从句修饰前面的整个主句，代替主句所表示的整体概念或部分概念。在这种从句中，which可以作主语，也可以作宾语或表语，多数情况下意思是与and this 相似，例如：
He succeeded in the competition, which made his parents very happy.

it是人称代词,作主语或者宾语.但是从句中关系代词作主语和宾语时就不能使用it了，定语从句中引导词可以是who ，which，where，whom，that，但不能用it

Answer 2

语 法 讲 解 ——虚拟语气英语中有三种不同的语气：陈述语气，祈使语气和虚拟语气。虚拟语气表示动作或状态与事实相反，或不可能发生的情况。一．虚拟语气用于条件状语从句中1.表示与现在事实相反或不可能发生：条件状语从句：一般过去时（虚拟语气中be→were）主句用：would(should,could,might)+动词原形。Ifwehadtimenow,wewouldreaditagain.IfIwereyou,Iwouldworkhard.2.表示与过去事实相反或不可能发生：条件状语从句：had+过去分词；主句：would(should,could,might)+have+过去分词。Ifhehadtakenmyadvice,hewouldhavesucceededinthetest.IfIhadknownyourtelephonenumberthen,Iwouldhavecalledyou.3.表示与将来的事实可能相反或不可能发生：条件状语从句：①一般过去时②should+动词原形③wereto+动词原形；主句：would(should,could,might)+动词原形。Ifitshouldrain,thecropswouldbesaved.Ifitweretosnowtomorrow,theywouldnotgoout.P.S虚拟条件句的特殊情况(1).混合/错综型虚拟语气 当条件状语从句表示的行为和主句表示的行为所发生的时间不一致时，称为‘错综条件句’，动词的形式要根据它所表示的时间作出相应的调整。Ifyouhadfollowedmyadvice,youwouldbebetternow.(从句说的是过去，主句是现在)IfIwereyou,Iwouldhavetakenhisadvice.(从句是现在，主句是过去)（2）省略if的虚拟语气如果从句中含有were/should/had时，则可以把这三个词置于句首，省略if.采用倒装语序。Ifitshouldhappen,whatwouldyoudo?→Shouldithappen,whatwouldyoudo?Ifhehadrecognizedme,hewouldhavecomeover.→Hadherecognizedme,hewouldhavecomeover.(3)含蓄虚拟条件句有时候假设的情况不以if引导的条件从句形式表现出来，而是通过一个介词短语，连词或其他形式表示。常用的词或短语有：without,with,butfor(要是没有),otherwise,or,but等。Withoutyourhelp(=Ifwehadnothadyourhelp),wecouldnothavesucceeded.Butforelectricity(=Iftherewerenoelectricity,therewouldbenomodernindustry.Hefeltverytiredyesterday,or/otherwisehewouldhaveattendedtheparty.(4)在ifitwere(hadbeen)notfor….句型中的虚拟语气虚拟语气也常用于‘ifitwere/(hadbeen)notfor….’句型中，意为‘要不是由于…’。Ifitwerenotforpeace,wecouldnotbelivingahappylifetoday.Ifithadnotbeenforyourtimelyhelp,Iwouldhavegonebankrupt.二．虚拟语气用在名词性从句中1.主语从句中的虚拟语气这种虚拟语气主要用在itis+形容词（名词，过去分词）+that+主语+should+动词原形（should可以省略）这种结构中。常见的这类形容词有：advisable,anxious,better,desirable,surprising,natural,necessary,strange,important,regretful.常见的这类过去分词有：decided,demanded,recommended,requested,desired,required,ordered,suggested.常见的这类名词有：duty,pity,nosurprise,nowonder,regret,shame.Itwassuggestedthathe（should）jointheclubactivities.Itisstrange(surprising,disappointing)thatshe(should)nothavebeeninvited.Itisapitythathe(should)refusetoaccepttheoffer.2.宾语从句中的虚拟语气①wish后面所跟宾语从句谓语动词用虚拟语气（意为要是，但愿…就好了），其形式主要有三种：表示对现在情况的虚拟：wish后接一般过去时的从句表示对过去情况的虚拟：wish后接过去完成时/should，couldhavedone表示对将来情况的虚拟：wish后接would，could+动词原形Iwishitwerespringallyearround.Iwishitwouldbefinetomorrow.Shewishedshehadstayedathome.②在表示建议，命令，要求等动词后的从句要用(should)+动词原形。此类动词有：suggest,advise,propose,demand,require,insist（要求）,order,prefer,command,ask,request.Theteacherorderedthatallthebooks(should)behandedinontime.P.S.当suggest表示‘暗示，说明’之意；insist表示‘坚持认为，坚持说’之意时，从句不用虚拟，用正常的陈述语气。Themaninsistedthathehadneverstolenthemoney.Thesmileonhisfacesuggestedthathewassatisfiedwithlife.3.表语从句和同位语从句中的虚拟语气在表示建议，命令，要求的名词后的表语从句或同位语从句中的谓语动词要用（should）+动词原形。常见名词有：advice,desire,request,demand,proposal,suggestion,preference,requirement,order.Hegaveordersthattheguests(should)behospitalentertained.Hisproposalisthatwe(should)getridofthebadhabits.三．虚拟语气在其他情况中的使用1.在asif/asthough引导的状语从句中，若从句与事实相反，则要用虚拟语气。其形式如下：主语+asif/though+主语+一般过去时（从句的动作与主句同时发生）主语+asif/though+主语+would/could+动词原形（从句的动作相对主句来说在将来发生）主语+asif/though+主语+过去完成时（从句的动作先发生于主句动作之前）Hetalksasifheknewallaboutit.ShespeaksEnglishwellasifhehadstudiedinAmerica.HelearnsEnglishsohardasifhewouldgoabroad.2.在itis(high)timethat…句型中也用虚拟语气，从句常用过去式，有时也用should+动词原形，表示早该做而没做的事，含建议的意味。Itistimethatwewenthome.P.S.此句型中，注意与‘It/Thisisthe+序数词+timethat+现在完成式的区别。ThisisthefirsttimethatIhavebeenhere.3.wouldrather后接动词原形，但接从句时要用虚拟语气（宁愿，但愿）。Wouldrather+主语+一般过去时（表示与现在或将来相反）Wouldrather+主语+过去完成时（表示与过去相反）I’dratherhedidn’tgonow.I’dratheryouhadn’tdoneit.4.ifonly后接从句也用虚拟语气，其形式与wish的用法一致。5.动词hope,intend,mean,plan,want,think等词用过去完成的形式，表示的意思为‘本想，本打算，本认为’Hehadmeanttobuythefurniturelastweek,butitrained.（他上周本打算去买那件家具，但却下雨了）