一、It是代词
指除人以外的一切生物和事物，用来指代时间、季节、天气距离、环境。在这里，i是一个虚词，没有任何实际意义。例如:
①It is a quarter to eight now.
现在是八点差一刻。
②It is midnight. 现在是午夜。
③It is time for breakfast.
现在是吃早饭的时间。
④-What is the date today?今天几月几号?
--It is December 3rd. 十二月三号。
⑤--What season is it?现在是什么季节?
--It is summer. 夏季。
⑥--What is the weather like today?
今天天气如何?
--It is fine.天气很好。
--It is foggy and wet.天气多雾又潮湿。
⑧How far is it from the bus station to your home? 从汽车站到你们家有多远?
⑨It is around five miles to the shopping center.从这儿到购物中心大约有五英里。
二、It用作主语
用来指代文中出现过的人或事。
it此时可用来代替一个名词，一个短语，一个从句或一个句子。在句中可指生物,非生物，不明性别的人，也可以指一个动作或件事情(人物型)，还可以指人或事物的身份(指示型)。在这里,it是有实际意义的。例如:
①--What is this?这是什么?
--It is a computer.这是一台计算机。
②--What is that?那是什么?
--It is a map.那是一张地图。
③--Where is the dog?狗在哪儿?
--It is lying under the tree.它在树下面躺着呢。
④--Where is the post office?邮局在哪儿?
--It is near the hospital 在小医院附近。
⑤--Who is that man?那个人是谁?
--It is my brother.是我哥哥。
⑥He tried to get rid of smoking，but it was impossible.他尝试着戒烟，但那是不可能的。
⑦If you exercise every day ,it will keep you well and strong.如果你每天锻炼，你就会变得健康。
三、It作形式主语
为了避免将句中作真主语的不定式、动名词和名词子句置于句首造成头重脚轻，习惯上it可作为形式主语置于句首，指代后面的真正主语，使句子显得流畅。it 作形式主语的句型有:
It+谓语+不定式
It+谓语+动名词
It+谓语+名词性从句
1.不定式具有名词的性质，在句子中可作主语。例如:
It is wise to understand each other
=To understand each other is wise.
互相理解是明智的
记：把it翻译为中文时,it没有实际意义，要把后置的不定式作为主语来翻译。例如:
①It is necessary to have a good sleep before the test for us.
= To have a good sleep before the test for us is necessary.
对我们来说考试前好好睡一觉是很必要的。
②It is wrong to do so.=To do so is wrong.
这样做是错误的。
③It is a bad habit to eat junk food.
= To eat junk food is a bad habit.
吃垃圾食品是一种坏习惯。
④It took us 12 years to complete the project.
= To complete the project took us 12 years.完成这项工程花费了我们十二年的时间。
2.与不定式不同,动词-ing作主语时一般置于句子开端;置于句尾时则常将动词-ing改成不定式。因此用it代替动词-ing作形式主语的句子较少，主要用在以no use，useless, not any use, not much use, not any good, not much good, no good, no help, dangerous，senseless, worth ，while ,a waste 等词作主语补足语的句子中。例如:
①It is no use arguing with the people who won"t see reason.和不讲理的人争论是无用处的。
②Is it any good( any use ) making such a decision?做这样一个结论有用吗?
③It is dangerous playing with fire. 玩火是危险的。
④It is worth reading the book.
读这本书值得一读。
四、It可代替由that、连接代词或连接副词引导的名词性从句。
①It was clear that he had desire for peace.
= That he had desire for peace was clear.
很清楚他有和解的愿望。
②It was reported that the new president had been elected.
= Thatthe new president had been elected was reported.
据报道新总统已被选出来了。
③It is a pity that her sister cannot go shopping with us.
= That her sister cannot go skating with us is a pity.
她的姐姐不能和我们一起去购物,这太令人遗憾了。
五、It +名词性从句的结构一般可归为六类。
1.It+be动词+形容词+that从句
①It is strange/ obvious natural good/ possible/ funny/ surprising/usual +that从句。例如:
②It is strange that nobody knows where he lives.奇怪的是，谁也不知道他住在哪里。
2.It +be动词+名词+that从句
It is a fact/ an honor/a good thing/ good news/a pity + that从句。例如:
It is a pity that you didn"t come.
你没来，好遗憾。
3. It was said/reported/ announced/ expected/ decided + that从句。例如:
①It"s said that he has performed many miracles.据说他创造了许多奇迹。
②It was reported that muscle diseases were increasing rapidly.
据报道肌肉疾病正迅速增加。
4.It seems/happened/ turned out/occurred to me + that !从句。例如:
It seems that my teacher knows a lot about this.我的老师对这件事知道的似乎很多。
5. It is doubtful/a question/ uncertain/a mystery+连接代词或连接副词引导的子句。例如:
It is a question whether she would play the part. 她是否能扮演这个角色还是个问题。
6. It doesn"t matter/ doesn"t make too much difference +连接代词或连接副词引导的子句。例如:
It doesn"t matter whatever you do.
无论你做什么都没关系。
六、It的强调句型
句子结构：It+be动词+被强调的成分+子句。It此时是没有实意的虚词，它的作用是强调放在主语补语位置的某个句子成分。当强调部分指人时，可用who( whom)/that,其他情况下多用that,不能用which。
1.强调主语
①It was his brother who/ that broke the cup.是他的哥哥打碎了杯子。
②It was two pupils who/ that helped the blind man cross the street.是两个学生帮助这个盲人过马路。
③It is the people who/ that are really powerful.人民是真正有力量的。
④It is Professor Smith who/ that teaches us chemistry every Monday afternoon.是史密斯教授每周一下午教我们化学。
2.强调宾语
(1 )强调直接宾语。例如:
①It is chemistry that Professor Smith teaches us every Monday af-ternoon.史密斯教授每星期一下午是给我们讲化学。
②It was a New Year card that he sent his classmate.他寄给他同学的是张新年卡片。
(2)强调间接宾语。例如:
①It was his classmate whom/ that he sent a New Year card to.他将新年卡片寄给他的同学。
②It was the guard whom/that he showed the pass to.他将通行证出示给警卫。
③It is us whom/that Mr Li teaches chemistry every Monday afternoon.李老师每周下午给我们教化学。
8.强调状语
(1)强调时间状语。例如:
①It is every Monday afternoon that Mr Li teaches us Maths.李老师给我们讲数学课是在每星期一下午 。
②It was not until Saturday that he began to prepare the examination.他直到星期六才开始准备考试。
(2)强调地点状语。例如:
①It might have been on the bus that I lost my purse.
我可能在公共汽车上丢失了皮夹。
②It was before the fireplace that the old woman was sitting.
这位老妇人坐在壁炉前。
4.强调宾语补足语
①It was white that my aunt painted her bedroom.
我的姑姑把卧室漆成白色。
②It was captain that the team chose him.
队员们选他当队长。
③It was wrong that we considered him.
我们认为他是错误的。
④It is a fine player that we believe Jane.
我们相信珍妮是一个出色的选手。
5.强调主语补足语
It is Little Tiger that he is called.
他被称作小老虎。
七、It用在一些习语中作 主语
It cannot be helped.没有办法。
It is hard to say.很难说。
It comes to nothing.没有什么结果.
八、It作真正宾语
①She is very stubborn though she does not look like it.
她非常固执，虽然看上去并非如此。
②When he found what the world needed, Edison went ahead and tried to invent it.
当他发现世上需要的东西时，爱迪生总是尽力去发明它。
③You"d better have a try and you will find it quite interesting.你最好尝试一下，就会发现这很有趣。
九、It作形式宾语
1.代替不定式。例如:
①Mike found it interesting to read the book A Tale of Two Cities.
麦克发现读小说《双城记》很有意思。
②She made it her business to take care of her younger sister after her mother died.
在她妈妈死后，她认为照顾妹妹是自己的责任。
2.代替动词-ing。例如:
①He thinks it sheer waste of time arguing with a madman.他认为和一个疯子争论纯粹是浪费时间。
②Don"t you think it wrong wasting your time? 你不认为浪费时间是错误的吗?
3.代替that引导名词性从句。例如:
We should make it clear to the public that we are doing an im-portant job.
应向公众表明我们正在进行一项重要的工作。
I considered it true that he was ill.
我认为他病了，这是真的。
十、试题与解析
选择最佳答案填空。
1. Was it in 1969（）the American astronauts succeeded（）landing on the moon ?
A. when ;on
B. that;on ,a
C. when;in
D. that;in
2.（）in the park tomorrow.
A. It will lovely
B. It will be lovely
C. There will be
D. There will
3. In fact,（）is a hard job for the police to keep order in an important football match.
A. this B. that C. there D. it
4. How much does（ ）to rent a car?
A. it costs B. costs C. it D. it cost
5. What a mistake（ ） to come here!
A. it B.is,era C. is it D. it is
6.（ ）easy thing to master a skill.
A.It is not a
B.It not is a
C.It is not an
D. It not is an
7.（ ）that Miss Smith knows a lot about history.
A. It seemed
B. It seems
c. It is seemed
D. It was
8.（）too much difference who is in charge of the supermarket.
A. It doesn"t make
B. It is make
c. It doesn"t made
D. It is made
9. It（ ）in the morning , but now it has cleared up.
A. is cloudy
B. was cloudy
c. was cloud
D. is cloud
10. I hate（）when people talk with their mouths full.
A. it B. that C. these D. them
11. It（）when the new road will be opened to traffic.
A. is made clear
B. had be
c. made clear
D. was made clear
12.（）to Mr. Jackson how he became a writer.
A. It has mystery
B. It is mystery
c. It has a mystery
D. It is a mystery
13. People（）that the earth is center of the universe many
years ago.
A. took this for granted
B. took it for granted
c. took that for granted
D. took its for granted
14. It looks as if（ ）going to rain.
A. it were
B. there is
с. it is
D. there are
15.（）whether he will visit that country.
A. It is not decided
B. It decided not
с. It not decided
D. It was not decided
16. It ’s that he has retired.
A. is appeared
B. appearing
C. was appeared
D. appears
17. --Wasn"t it Dr. Want who spoke to you just now?
--（ ）.
A. I didn"t know he was
в. Yes, it was
с. No , he wasn"t
D. Yes ,he did
18.（ ）a long time since he last visited his hometown.
A.It is
в. It was
с. They are
D. They have
19.（ ）to use our time wisely.
A. We are good
в. It is important for us
с. We are important
D. It is important to us
20.（ ）going shopping on Sundays.
A. It is pleasure
в. It is a pleasant
C.It is a great pleasure
D. It is a great pleasant.
答案：
1 ~ 5. DBDDD
6~ 10. CВABA
11 ~ 15. ADBCA
16 ~ 20. DBABC