一、人称代词
1，it的最基本用法是作代词，主要指刚提到的事物，以避免重复:
①They watched the train until it disappeared in the distance.
2.，也可以指动物或婴儿(未知性别的婴儿或孩子):
②Is this your dog？No, it isn’t.
③They got a baby and it was a ten-pounder
3.，也可指抽象事物或指抽象环境和情景:
③I hate it when people talk with a full mouth.．
二、.非人称代词
1.it有时并不指具体的东西而泛指天气、时间、日期、距离、价值、度量、温度、环境等：
⑴.指天气：It is a lovely day, isn’t it?
⑵.指时间: It was nearly midnight when she came back.
⑶.指日期:It is April First today.
⑷.指距离:It is some 3000 kilometers from Beijing to Guangzhou.
⑸.指价值:It is three dollars.
⑹.指温度:Today it is 30 degrees centigrade.
三、其他用法
1.在句子的主语不太明确时充当主语，表示谁在做某事：
①Who is it there? It's I (me/you/he.....).
②I thought it was Mary, but it was not she.
③Her face lighted when she saw who it was.
2.泛泛的指某件事： (有时泛指一般情况)
①It doesn’t matter.
②It is a shame, isn’t it?
③How is it going?(情况怎样)
④It says in the newspaper that......
3.it用在一些词组中，it 没有特别的意思
The last train's gone. Come on, we'll foot it.(来，咱们步行吧。)
四、作形式主语，替代主语从句，动词不定式，或动名词短语：
1.作形式主语替代主语从句
⑴It is clear ( obvious， true， possible， certain....) that 从句 常译为＂┅清楚的（显然的，真的，可能的,肯定的．．．）＂
It is very clear that he’s round and tall like a tree.
= That he’s round and tall like a tree is very clear.
⑵It is important ( necessary， right， strange， natural...) that 从句 常译为┅是重要的(必要的,对的,奇怪的,自然的┅).that 后的从句中要用虚拟语气（should 动词原形），should 可以省去，建议记住该句型中的形容词。
①It is important that we (should) learn English well.
②It is necessary that he (should) remember these words.
⑶It is said (reported/ learned/believed/thought/known/told/hoped.....) that 从句 常译为＂据说（据报道，据悉．．．）＂。
①It is said that he has come to Beijing.
②It is reported that another earth satellite has been put into orbit.
⑷It is suggested ( advised/ ordered / demanded/ insisted/ commanded... ) that 从句.that 后的从句要用虚拟语气（should 动词原形），should 可以省；常译为＂据建议；有命令．．．）
①It is suggested that the meeting ( should ) be put off.
②It was ordered that we ( should ) arrive there in two hours.
⑸It is time ( about time ，high time ) that从句(虚拟语气:动词用过去时did)
① It is time that children went to bed.
② It is time you bought a new car.
③ It is (high ) time you made up your mind.
⑹It is the first ( second ... ) time that从句(从句用现在完成时 have done )
It was the first ( second ... ) time that从句(从句用过去完成时had done )
常译为＂是第一（二）．．．次．．．＂。
It is the first time I have been here. = This is the first time I have been here
⑺It is a pity ( a shame /an honour/a good thing/a fact,/a surprise/... ) that从句.
that后的从句一般用虚拟语气（should 动词原形），should可省去．表示出乎意料，常译为＂竟然＂。没有这种意义时，则不用虚拟语气。
①It is a pity that such a thing ( should ) happen in your class. 这种事竟然发生在你们班 上，真是遗憾！
②It is a pity that he is ill. 他生病了，真遗憾！
⑻It happens (seems， looks， appears ) that从句.常译为 “ 碰巧…,似乎是…,看起来…”
①It happened ( so happened ) that he met his teacher in the street. 碰巧．．.
②It seems that he will be back in a few days. 看来．．．
2.作形式主语替代不定式
. ⑴ It is kind ( of sb. ) to do sth. 不定式的逻辑主语是由 of引起，主句中的形容词必须是能表示逻辑主语特征的褒义或贬义形容词。 常见的词有：
bad ， brave ， careless， clever ， cruel ， foolish ， good （好心的）， honest ， horrible ， kind ， lazy ， modest ， naughty ， nice（有教养的）， polite， rude ， silly ， stupid ， wise ， wrong（错误的）等。
这个句型可以改写为：sb. is kind to do sth. 。
如:It is kind of you to say so. = You are kind to say so.
⑵It is necessary ( for sb. ) to do sth. .不定式的逻辑主语是由for引起，主句中的形容词通常是表示重要性，紧迫性，频繁程度，难易，安全等情况的中性形容词。 常见的形容词有：
important， necessary， natural easy ， safe ， common ， normal ， hard ， difficult ， dangerous ， unusual， rare ， impossible ， pleasant
如:It is important for her to come to the party. = It is important that she (should ) come to the party.
⑶It takes sb. ... to do sth. 常译为＂做．．．要花费某人．．．＂。
如:It took thousands of people many years to build the Great Wall.
3.作形式主语替代动名词短语
It is no good / no use / useless doing sth. 常译为 “┅有好处或没有用”
①It is no good learning English without speaking English.
②It's useless trying to argue with Shylock.
五、作形式宾语，代替不定式，动名词短语或宾语从句。
We think it important to learn a foreign language.
该句型中的it 作形式宾语，该结构中常用的动词有：think, believe, make ,find consider, feel； 如:
We think it our duty to clean our classroom every day.
He felt it important learning English well.
They found it difficult that they would finish their work in two days.
The Internet makes it easier for companies to keep in touch with customers.
六、.it的重要句型
1.强调句型: It is/was 被强调部分 that 从句 (被强调的主语如果是人，that可以由who换用)
①It was about 600 years ago that the first clock with a face and an hour hand was made.
②It was they that (who ) cleaned the classroom yesterday.
③It was in the street that I met her father.
④It was yesterday that I met her in the street.
⑤It is you that /who are wrong.
特例:It is not until 被强调部分 that ... 该句型也是强调句型。主要用于强凋时间状语，译成汉语＂直到．．．才．．．＂，可以说是 not ... until ... 的强调形式。
It was not until she took off her dark glasses that I realized she was a famous film star.
= Not until she took off her dark glasses did I realize she was a famous film star.
= I didn’t realize she was a famous film star until she took off her dark glasses.