一、 代替上文中提到过的人或事物。指代人时，一般用于小孩或身份不十分明朗的人。
1) －Who is the baby? －It‘s my teacher’s son．
2) －Who is that gentleman? －It's my friend Tom．He(不可用It) wants tosee you．
不定代词one 指代前文提到的同类事物之一，而 it 特指上文中提到的同一事物。试对比：
This pair of shoes is small. May I have another one？（泛指的另一双）
这又鞋子太小了，再拿一双给我看看好吗
My pen is missing.Ican’td find it.
我的钢笔丢了找不到了(it指my pen)
二. it 作为无人称动词的主语，表示天气 ，时间，距离，环境情况等等。这时的it不指某个具体的东西。没有明确的含义。译成汉语可不必译出来。如
1).It‘s half an hour’s walk from here to our school．
从这儿到我们学校是步行一个小时的路程
2) It‘s nice and warm here．
这里非常暖和
3) But it's two o'clock now, and it's time for us to go toschool．
两点了该是上学的时间了
4) It’s quite close in the room . Let’s open the window for awhile。
房间里太闷，把窗子打开一会儿吧。
三.“It” 可以代替指示代记词This ,that .
例：---Whose dictionary is that ?---It is mine
四,用作形式主语或形式宾语。
1. 当作主语的不定式、－ing短语或从句太长时，往往用it作形式主语，而将真正的主语移至句末，
以避免句子头重脚轻。it作形式主语的常见句型：
★it替代动词不定式作主语的常见句型为:
1) It is necessary /important /possible/easy…..for sb to do….
如:It is necessany to learn a foreign lanuage.学一门外语是有必要的。
It is kind /foolish/polite/brave/careless…of sb todo.常用形容词有:clever/wise/bright/silly/foolish/stupid/kind/cruel/right/wrong等如:
It is kind of you to help me=You are kind to help me.:谢谢你帮助我.
3)It is up to sb to do sth.该由某人做某事如:It is up to you todecide.由你来决定.
4)It is (not)like sb to do sth.做某事是(不)是某人的行为.如:It was not like youto have said such rude words.
说出这样粗鲁的话不像你的行为.
5)It is one’s turn to do sth.轮到某人做某事.如;It is your turn to speak轮到你发言了.
6) It takes time to do sth .如:It took me an hour to finish readingthe novel.读完这本小说花了我一个小时
★It替代动名词作主语的常见句型：
It’s no good/use doing… 如:It’s no use / good crying over spiltmilk．覆水难收
It’s (well)worth (one’s )while doing/to dosth.
It’s worth（your）while to learn english。学英语是值得的.
★It替代主语从句常见句型:
1） It is a pity (a shame ,a fact ,a wonder ,a good thing ,good newsetc)that……例如：
It is a shame that he didn’t pass the exam.
很遗憾他没有通过考试。
2） It is strange(natural surprising obvious true good wonderfulpossible,impossible,likely,unlikely,quite clear.certain ,etc) that…。例如：
It was quite clear that they had no disire for peace.
很清楚他们没有和平的诚意。
3） It seems (happened ,turned out, occurred to sbetc.)that…例如：
It seemed that things were not as they expected.
看起来情况并不像他们所想象的那样。
4） It is said (reported ,decided ,expected ,arranged ,rumoured,etc)that …例如：
It is said that the tickets have been sold out .
据说票已售完。
5) It is time that 从句（谓语用过去时）
It is time that he went to school.他本该去上学了.
6)It seems/appears/sounds/looksas if…….似乎/听起来/看起来….
It seems as if it is going to rain.仿佛要下雨了.
2当复合宾语中的宾语是不定式、－ing短语或从句，往往用it作形式宾语，而将真正的宾语移至句末，以使句子重心后移。
Do you think it a good habit to read in bed?
你认为在床上看书的习惯吗？
We find it difficult to learn English well in such a shorttime．
我们认为很难在这如此短的时间内学好英语
I will leave it to you to buy the ticket.
我要把买票的事委托给你
We think it no use crying over spilt milk．
我认为木已成舟哭也没用。
You may depend on it that they will support you.
你相信好了，他们会支持你的。
They will answer for it that the computer is reliable.
他们愿意保证这台电脑是可靠的。
We made it clear when and where we were going to have themeeting．
我们宣布了何时何地要举行会议。
I tookit for granted that you would stay with us .
我信以为真你要和我们呆在一起.
五.It用于强调结构：it没有具体意义，可强调谓语动词外的成分。其基本句型为：
It+be+被强调部分+that(who)+其它……。
★强调主语：
主语是人，一般用who （也可用that）.其他情况用that.。强调主语用主格；who或that后面的句子中谓语动词与被强调的主语在数和人称上应保持一致。例如：
It is I who (that) am to blame
应付责任的是我。
It was you who didn’t keep your promise。
是你不遵守诺言
It was long overwork that made them get tired.
长期超负荷的劳动使他们疲被倦不堪。
★强调宾语：当宾语是指人的名词或代词时、强调句中的连接词用whom或that，其他情况用that。如：
It is you that we have been wanting to visit.
我们一直想去访问的是你。
★强调状语：被强调的状语可以是时间状语或地点状语，原因状语。强调句中的连接词只用that例如：
It was at that time that I thought of giring him a call.
直到那时我才想起要给他一个电话。
It was in the zoo that they saw the elephants.
他们是在动物园见的大象。
在运用it引导的强调句时，要注意以下几点：
1） 强调句的谓语is或was永远用单数，例如：
It was yesterday that I met your father in the street．
2）不管被强调部分是哪种状语，只能用that连接，不得使用where、when等连词。例如：
It was in the street that I met your father yesterday．
3）被强调部分为人时，可用who(主格)和whom(宾格)代替that，用that也行。例如：
It was your father that/whom I met in the street yesterday．
It was I that/who met your father in the street yesterday
4）强调句中 也要注意主谓一致、人称一致和否定转移的问题。例如：
It is they who are our friends．
5）not …untill…句型的强调形式为It is/was not untill that 例如:
It was not until ten o'clock that we got home last night．
6)as since because 均可引导原因状语从句,但是强调句型中只可以强调because从句例如:
It was because he was ill that he didn’t come here.
7)注意不要混淆强调句和其他从句。It iswas…that…为强调句标志，去掉后仍可组成一个完整的句子，只是没被强,调,而其他从句一般不具备这一特征。
比较：
1) It was ten o'clock when we got home lastnight．我们昨晚到家时已十点了。
2) It was at ten o'clock that we got home lastnight．我们昨晚是在十点到家的。
3) It was not until ten o'clock that we got home last night．
8)强调句型的一般疑问句形式为: “Is/was＋it+被强调部分＋that/who/whom+其它?
其特殊疑问句形式为:Wh-疑问词/How+is/was+it+that” +其它?
what is it that you want to say ？
who was it that came here this morning！
六
minutes before…表示 …去….就 /才“或”不多久就…
例： It was not long before the whole country rose up and drove theAustrian soldiers from their homeland.
It is …since 从句意思是；“自从。。。。以来已经多久了”。如果since从句中的谓语动词是短暂性动词，那么就表示该动作的开始，如果从句中的谓语动词是延续性动词或表示求态的动词，则指动作或求态的结束。
就学生而言：It is nearly 12 years since I became a stndent.
就老师而言：It is nearly 12 years since I was a stndent.
It’s the first/second/third time that+主语+ have/has+done/been…
主语第几次…(it 也可换成this that today等)如:
It’s the first time that I have come here.我第一次到这儿.
七、it 用于某些句语中，it 常用作某些动词的含糊宾语。例如：
That’s it.对了.
I can’t help it.=It can’t be helped.我无能为力.
It’s the same with sb.某人的情况也是一样.
Get with it ! 振作精神！
Keep at it ! 坚持下去！
Go it while you aire young.
趁你还年轻就努力干吧。
make it 规定时间，做到,成功。
Foot it 步行去
As it is/was 事实上。
Fight it ont 决一雌雄
keep at it 别松劲
Catch it 冒险，碰运气
Leg it 逃走
Brave it 敢干，抵抗
He thinks he is it .他自以为了不起。
Why ,among scientists,he is it.嗨，在科学家中他最出色。
巩固练习
1Was it during the Second World War ____ he died？(NMET88)
A．that B．While C．in which D．Then
2．Is ____ possible to fly to the moon in a spaceship？(NMET88)
A．now B．that C．it D．Man
3.I don't think ___ possible to master a foreign language withoutmuch momory work．(NMET90)
A．This B．that C．its D．It .
4. Does ___matter if he can’t finish the job on time?(NMET91)
A. This B.That C.He D.It
5.It was not ___she took off her dark glasses___Irealiced she was afamous film star. (NMET91)
A.When ,that B.Until,that C.Until.when D.When,then
6.It worried her a bit ___her hair was turning grey.([NMET92]
A. Whit B.That C.If D.For
7.---What was the party line? ---Wonderful. It’s years___Ienjoyedmyself to much.( NMET 93)
A. After B.Before C.when D.Since
8.Tom,s mother kept telling him that he should workharder,but___didn,t help. ,( NMET 93)
A. He B. Which C. She D. It
9.__is a fact that English is being accepted as an internationallangnage. (NMET 94)
A. There B.This C. That D. It
10..It was about 600 years ago ___ the first clock with face and anhour hand w was made (NMET97)
A．That B．Until C．before D．when
11.______computers play such an important part in our dailylife?
A.Why is it that B.Why it is that C.Why was it that D.Why isit
12. It was only when I reread his poems recently ___ I began toappreciate their beauty．(NMET98)
A．until B．that C．then D．so
13.,I hate____when people talk with their mouthsfull．,(NMET98)
A it B．that C．these D．them
14．Onford,as we know___,is one the best unirersities in theworld.
A. that B.it c.this D.不填.
15.It was the third time I____to that city.
A.have been B had been C.came D. Was .
16.When was it____you arrived in New York?
A.that B.at which C.before D. Since.
17.It was____I met Mr.Green in Shanghai.
A. ten years since B.five years C.two months when D. many years agothat
18.Jack is ill, have you heard about____?
A. that B. this C.it D.which
19___who suggested that he____to New York in order to get a directfight.
A.It was her …went B. It was her …go C. It was she …went D.It wasshe …go
20___has already been pointed out , grammar is not a set of deadrules.
A . As. B.It C.That D. This
1A这是强调句一般疑问句形式
2.C.此处it代替不定式to fly
3.D it做形式宾语代替to master
4.D.it 表示某种情况
5.B.此处考察It is not untill that 句型
6.B.It代表that从句
7.D该题为情景对话.题意为我好长时间没有玩得如此愉快了.
8.D.It 代替Tom’smother…….harder这件事
9.D.It 代表that从句
10.A. It was that 为强调句型
11A.该题为强调句型的特殊疑问式形式
12.B.该题为强调句型强调when引导的状语从句
13.A.It代替when从句该句型的特点为:I hate/like/dislike /love+it+when从句.
14.D.as we know是插入语.是非限定性定语从句
15.B.该句型后半部分常用完成时态.前如果it is后用现在完成时前如果是it was后用过去完成时.
16.A. 该题为强调句型的特殊疑问式形式
17D. It was that 为强调句型入选A后应为过去完成时态BC两答案ten years/two months为一段时间不可和met连用
18.C.it 代替Jzck is ill这件事.
19.D.该题考察强调句型强调主语用主格.同时考察suggest that后动词用(should)+do.
20.A.AS引导非限定性定语从句代表Gramma ris not a set of deadrules这一点.如用it后需有that.
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