一、it 作人稱代詞的用法
1.指事物
作為人稱代詞，it 可以除人以外的一切事物或動物。如：
I dropped my watch and it broke.
我把手表掉在地上摔壞了。
It's hard work, but I enjoy it.
工作很辛苦，可是我樂意干。
“Where is the dog?”“It’s in the bedroom. ”
“狗在哪?”“在臥室里”。
2.指人
it 指人主要用于指不性別不明的嬰兒或用于確認某人的身份。如：
Is it a boy or a girl?
是男孩還是女孩?
There is a knock on the door. It must be the postman.
有人在敲門，一定是郵遞員。
【說明】在答語中，常用來指本人，如說It's me。
3.代替某些代詞
代詞it 還可用于代替指示代詞this, that 以及復(fù)合不定代詞 something, anything, nothing等。如：
“What's this?”“It's a new machine. ”
“這是什么?”“是一種新機器”。
Nothing is wrong, is it?
沒出什么問題，是嗎?
二、it 作非人稱代詞的用法
1.基本用法
it 作非人稱代詞的用法，主要用于指時間、距離、價值、天氣、氣候及溫度等自然現(xiàn)象。如：
It's too late to go there now.
現(xiàn)在去那兒已經(jīng)太遲了。
It rained all day yesterday.
昨天下了一天的雨。
It can get very hot here.
這里有時會很熱。
2.用于某些句型
It's time for sth.
該做某事了。
It's time to do sth.
該做某事的時候了。
It's time for sb to do sth.
某人該干某事了。
It's (about / high) time + that-從句.
某人該做某事了。
(從句謂語用過去式，有時也用“should+動詞原形”)
It's first (second) time + that-從句.
某人第幾次干某事。
(從句謂語用現(xiàn)在完成時)
It's + 時間段 + since-從句.
自從……有一段時間了。
It's + 時間段 + before-從句.
過多長的時間才……
三、it用作形式主語
一、主語從句是在復(fù)合句中充當(dāng)主語的從句，通常放在主句謂語動詞之前或由形式主語it代替，而本身放在句子末尾。
1. It 作形式主語和it引導(dǎo)強調(diào)句的比較：
It 作形式主語代替主語從句，主要是為了平衡句子結(jié)構(gòu)，主語從句的連接詞沒有變化。而it引導(dǎo)的強調(diào)句則是對句子某一部分進行強調(diào)，無論強調(diào)的是什么成分，都可用連詞that。被強調(diào)部分指人是也可用who/whom。例如：
It is a pity that you didn't go to see the film.
你沒有去看這個電影，多可惜啊！
It doesn't interest me whether you succeed or not.
對于你是否成功，我并不感興趣。
It is in the morning that the murder took place.
兇殺案發(fā)生的時間是在早上。
It is John that broke the window.
打碎窗戶的是約翰。
2.用it 作形式主語的結(jié)構(gòu)
(1)It is ＋名詞＋從句
It is a fact that … 事實是…
It is an honor that …非常榮幸
It is common knowledge that …是常識
It's a pity that you missed the exciting football match.
多么可惜啊，你錯過了一場精彩的足球比賽。
(2)it is ＋形容詞＋從句
It is natural that… 很自然…
It is strange that… 奇怪的是…
It is uncertain whether he can come to Jenny's birthday party or not.
他來不來參加簡的生日派對，這個不確定。
(3)it is ＋不及物動詞＋從句
It seems that… 似乎…
It happened that… 碰巧…
It seemed that he didn't tell the truth.
似乎他沒有講真話。
It happened that I was out when he called.
碰巧，他打電話的時候我在外面。
(4)it＋過去分詞＋從句
It is reported that… 據(jù)報道…
It has been proved that… 已證實…
It is reported that 16 people were killed in the earthquake.
據(jù)報道，在這場地震中，有16人遇難。
常用句型如下：
（1）It + be + 名詞+ that從句
It is common knowledge that 是常識
It is a surprise that 令人驚奇的是
It is a fact that 事實是
（2）It + be + 形容詞 + that從句
It is necessary that 有必要
It is important that 重要的是
It is obvious that 很明顯
（3）It + be + 動詞的過去分詞+ that從句
It is believed that 人們相信
It is known to all that 眾所周知
It has been decided that 已決定
（4）It + 不及物動詞 + that 從句
It appears that 似乎
It happens that 碰巧
It occurred to me that 我突然想起
另注意：在主語從句中用來表示驚奇、不相信、惋惜、理應(yīng)如此等語氣時，謂語動詞要用虛擬語氣“(should) +do”，常用的句型有：
It is necessary (important, natural, strange, etc.) that
It is suggested (requested, proposed, desired, etc.) that
