壹、人稱代詞
1，it的最基本用法是作代詞，主要指剛提到的事物，以避免重復:
①They watched the train until it disappeared in the distance.
2.，也可以指動物或嬰兒(未知性別的嬰兒或孩子):
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環境和情景:
③I hate it when people talk with a full mouth.．
二、.非人稱代詞
1.it有時並不指具體的東西而泛指天氣、時間、日期、距離、價值、度量、溫度、環境等：
⑴.指天氣：It is a lovely day, isn’t it? ⑵.指時間: It was nearly midnight when she came back.
⑶.指日期:It is April First today. ⑷.指距離:It is some 3000 kilometers from A to B.
⑸.指價值:It is three dollars. ⑹.指溫度:Today it is 30 degrees centigrade.
三、其他用法
1.在句子的主語不太明確時充當主語，表示誰在做某事：
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： (有時泛指壹般情況)
①It doesn’t matter. ②It is a shame, isn’t it? ③How is it going?(情況怎樣)
④It says in the newspaper that......
3.it用在壹些詞組中，it 沒有特別的意思
The last train's gone. Come on, we'll foot it.(來，咱們步行吧。)
四、作形式主語，替代主語從句，動詞不定式，或動名詞短語：
1.作形式主語替代主語從句
⑴It is clear ( obvious， true， possible， certain....) that 從句 常譯為＂┅清楚的（顯然的，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 從句 常譯為┅是重要的(必要的,對的,奇怪的,自然的┅).that 後的從句中要用虛擬語氣（should + 動詞原形），should 可以省去，建議記住該句型中的形容詞。
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 從句 常譯為＂據說（據報道，據悉．．．）＂。
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 從句.that 後的從句要用虛擬語氣（should + 動詞原形），should 可以省；常譯為＂據建議；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從句(虛擬語氣:動詞用過去時did)
① It is time that children went to bed.
⑹It is the first ( second ... ) time that從句(從句用現在完成時 have done )
It was …(從句用過去完成時had done )常譯為＂是第壹（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從句.
that後的從句壹般用虛擬語氣（should + 動詞原形），should可省去．表示出乎意料，常譯為＂竟然＂。沒有這種意義時，則不用虛擬語氣。
①It is a pity that such a thing ( should ) happen in your class. 這種事竟然發生在妳們班上，真是遺憾！
②It is a pity that he is ill. 他生病了，真遺憾！
⑻It happens (seems， looks， appears ) that從句.常譯為 “ 碰巧…,似乎是…,看起來…”
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. 看來．．．
2.作形式主語替代不定式
. ⑴ It is kind ( of sb. ) to do sth. 不定式的邏輯主語是由 of引起，主句中的形容詞必須是能表示邏輯主語特征的褒義或貶義形容詞。 常見的詞有：
bad ， brave ， careless， clever ， cruel ， foolish ， good （好心的）， honest ， horrible ， kind ， lazy ， modest ， naughty ， nice（有教養的）， polite， rude ， silly ， stupid ， wise ， wrong（錯誤的）等。 這個句型可以改寫為：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. .不定式的邏輯主語是由for引起，主句中的形容詞通常是表示重要性，緊迫性，頻繁程度，難易，安全等情況的中性形容詞。 常見的形容詞有：
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. 常譯為＂做．．．要花費某人．．．＂。
如:It took thousands of people many years to build the Great Wall.
3.作形式主語替代動名詞短語
It is no good / no use / useless doing sth. 常譯為 “┅有好處或沒有用”
①It is no good learning English without speaking English.
②It's useless trying to argue with Shylock.
五、作形式賓語，代替不定式，動名詞短語或賓語從句。
We think it important to learn a foreign language.
該句型中的it 作形式賓語，該結構中常用的動詞有：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.
CF: keep sth, in mind / keep in mind that
六、.it的重要句型
1.強調句型: It is/was + 被強調部分 + that 從句 (被強調的主語如果是人，that可以由who換用)
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.
特例:It is not until + 被強調部分 + that ... 該句型也是強調句型。主要用於強雕時間狀語，譯成漢語＂直到．．．才．．．＂，可以說是 not ... until ... 的強調形式。
It was not until she took off her dark glasses that I realized she was a famous film star.