一、it 作人称代词的用法
1.指事物
作为人称代词，it 可以除人以外的一切事物或动物。如：
I dropped my watch and it broke.
我把手表掉在地上摔坏了。
It's hard work, but I enjoy it.
工作很辛苦，可是我乐意干。
“Where is the dog?”“It’s in the bedroom. ”
“狗在哪?”“在卧室里”。
2.指人
it 指人主要用于指不性别不明的婴儿或用于确认某人的身份。如：
Is it a boy or a girl?
是男孩还是女孩?
There is a knock on the door. It must be the postman.
有人在敲门，一定是邮递员。
【说明】在答语中，常用来指本人，如说It's me。
3.代替某些代词
代词it 还可用于代替指示代词this, that 以及复合不定代词 something, anything, nothing等。如：
“What's this?”“It's a new machine. ”
“这是什么?”“是一种新机器”。
Nothing is wrong, is it?
没出什么问题，是吗?
二、it 作非人称代词的用法
1.基本用法
it 作非人称代词的用法，主要用于指时间、距离、价值、天气、气候及温度等自然现象。如：
It's too late to go there now.
现在去那儿已经太迟了。
It rained all day yesterday.
昨天下了一天的雨。
It can get very hot here.
这里有时会很热。
2.用于某些句型
It's time for sth.
该做某事了。
It's time to do sth.
该做某事的时候了。
It's time for sb to do sth.
某人该干某事了。
It's (about / high) time + that-从句.
某人该做某事了。
(从句谓语用过去式，有时也用“should+动词原形”)
It's first (second) time + that-从句.
某人第几次干某事。
(从句谓语用现在完成时)
It's + 时间段 + since-从句.
自从……有一段时间了。
It's + 时间段 + before-从句.
过多长的时间才……
三、it用作形式主语
一、主语从句是在复合句中充当主语的从句，通常放在主句谓语动词之前或由形式主语it代替，而本身放在句子末尾。
1. It 作形式主语和it引导强调句的比较：
It 作形式主语代替主语从句，主要是为了平衡句子结构，主语从句的连接词没有变化。而it引导的强调句则是对句子某一部分进行强调，无论强调的是什么成分，都可用连词that。被强调部分指人是也可用who/whom。例如：
It is a pity that you didn't go to see the film.
你没有去看这个电影，多可惜啊！
It doesn't interest me whether you succeed or not.
对于你是否成功，我并不感兴趣。
It is in the morning that the murder took place.
凶杀案发生的时间是在早上。
It is John that broke the window.
打碎窗户的是约翰。
2.用it 作形式主语的结构
(1)It is ＋名词＋从句
It is a fact that … 事实是…
It is an honor that …非常荣幸
It is common knowledge that …是常识
It's a pity that you missed the exciting football match.
多么可惜啊，你错过了一场精彩的足球比赛。
(2)it is ＋形容词＋从句
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.
他来不来参加简的生日派对，这个不确定。
(3)it is ＋不及物动词＋从句
It seems that… 似乎…
It happened that… 碰巧…
It seemed that he didn't tell the truth.
似乎他没有讲真话。
It happened that I was out when he called.
碰巧，他打电话的时候我在外面。
(4)it＋过去分词＋从句
It is reported that… 据报道…
It has been proved that… 已证实…
It is reported that 16 people were killed in the earthquake.
据报道，在这场地震中，有16人遇难。
常用句型如下：
（1）It + be + 名词+ that从句
It is common knowledge that 是常识
It is a surprise that 令人惊奇的是
It is a fact that 事实是
（2）It + be + 形容词 + that从句
It is necessary that 有必要
It is important that 重要的是
It is obvious that 很明显
（3）It + be + 动词的过去分词+ that从句
It is believed that 人们相信
It is known to all that 众所周知
It has been decided that 已决定
（4）It + 不及物动词 + that 从句
It appears that 似乎
It happens that 碰巧
It occurred to me that 我突然想起
另注意：在主语从句中用来表示惊奇、不相信、惋惜、理应如此等语气时，谓语动词要用虚拟语气“(should) +do”，常用的句型有：
It is necessary (important, natural, strange, etc.) that
It is suggested (requested, proposed, desired, etc.) that
