1．it代表刚提到的东西（可指具体东西，也可指抽象东西），也可指一个彼此都知其所指的东西。如：
You have saved my life；I shall never forget it．你救了我的性命，我永远不会忘记。（it指“救命”这件事）
You can't eat the cake and it's mine．你不能吃这蛋糕，这是我的。（it指蛋糕，具体事物）
How is it going with you？你的情况如何？（it指双方都知晓的事情）
2．it用来表示时间、天气、自然环境、距离等
1）指时间：
It was 12 o'clock when they came out ofthe hall．他们从大厅走出来的时候已经12点钟了。
2）指天气：
It's damp and cold．I think it's going to rain．天气又潮又冷，我想
要下雨了。
3）指自然环境：
It'll be lovely in the garden tonight．今晚花园将会很秀丽。
4）指距离：
How far is it from your office to the bank？你办公室离那家银行有多远？
3．指代不定式和动名词
It makes me sick to think about the problem．考虑这个问题令我很难受。（代替不定式to think about the problem）
It's terribly tiring working late like this．工作得这么晚真是太累了（代替动名词working late like this）。常见用it代替动名词的结构有：It's no use ／no good ／no need ／hopeless ／worth while doing sth．如：It's no good waiting this morning．Come back Monday．今天上午等待是没有意义的。下周一再来。
4．用for和of引起的短语，表示动作逻辑上的主语
It's very kind of you to have listened to me．你听了我的话真好。It's important for us students to learn English well．对我们学生来说学好英语十分重要。
5．代替that引起的从句，（注意几种不同情况）
It was clear that he has known the news．很清楚他已经知道消息了。
It's such a GREat pleasure that you are here．你在这里真令人高兴。
It suddenly occured to him that this was an act he could never do．他突然想起这里一个他永远不能做的行为。
注：这种从句中的谓语有时需用虚拟语气。从句常用should（也可省略），表示“应该”或“竟然”。列举如下：
It was necessary that he should work hard．他努力工作是十分必要的。
It was strange that she（should）order her mother．真奇怪她竟然
命令其母亲。
6．指代连接代词或副词引起的名词性从句
Is it known where they went？知道他们去哪里了吗？
It's not my business how she chooses to live．她如何选择生活不关我的事。
7．用于强调句。强调主语，宾语，介词宾语以及状语。（强调人时用that和who ／whom皆可）
It was she who ／that had been wrong．是她错了。（主语）
It was the girl whom ／that I met just now．我刚才遇见的就是这个女孩。（宾语）
It was Tom to whom the teacher had talked．老师与其谈话的那个人是汤姆。（介词宾语）
It was on Monday night that all this happened．所有这一切就是在星期一的晚上发生的。（状语）
8．作形式宾语（代替不定式和从句）
I find it easy enough to get on with Kate．我发觉与凯特相处很容易。（代替不定式）
I think it best that you should stay here．我认为你最好留在这里。（代从句）
