一、作人称代词，可用来代替人、物或事。—Who is knockingat the door?—It's me谁在敲门？是我 。
The ant is not gathering this food for itself alone. Ithas two stomachs in its body.蚂蚁不只 是为自己采食。 它的身体里有两个胃。
When service is interrupted, as it was by last night'sstorm, John has to inform the pu blic what the company isdoing about the problem.每当供电中断时，就像昨夜因暴风雨而造成的那样 ，约翰就必须通知大家，公司为解决这个问题正在做些什么。
二、作非人称代词，表示时间、天气、季节、距离、环境等。
It is tea time. Mrs Tuner goes into the kitchen.到了喝茶的时间了，特纳夫人走进了厨房。
It was winter. it was ten degree below zero.现在是冬天，气温是零下10度。
It is only half an hour walk to the tall tower. 只有半小时的路程就可以到达高塔了。
It will be lovely in the park today.今天公园里一定很好玩。
三、作先行词。
1.作形式主语。英语中不定式、动名词、主语从句在作主语时主语往往显得很长，使整 个句子看上去有点 头重脚轻，这样就借助“it”来作形式上的主语，而把真正的主语移到句 子后边去，使全句看上去平稳一些。 “it”没有实际的意义。
It is fun for her to study, but it is difficult to studyand work at the same time.对于她来说学 习是有趣的，可是要边工作边学习对她来说是困难的。
It is necessary for them to get an electrician to do thework for them.对他们来说请一位电工来为他们做这项工作是必要的。
It will be no good learning without practice. 学习而不实践是不好的。
It is no use talking to him about it.和他谈这事没用。
It is possible that they're finished the bridge.他们完成这座桥是可能的。
It is important that we should learn from each other andhelp each other.我们应当互相学习，互相帮助，这是很重要的。
2.作形式宾语。这种情况通常只用于能带以名词或形容词作宾语补语的动词后面。
In the 1870's, when Marx was already in his fifties, hefound it important to study the si tuation in Russia, so hebegan to learn Russian.在19世纪70年代，马克思已经五十几岁了，他觉得研 究俄国的形势很重要，便开始学习俄语。
This has made it necessary for agriculture and industryto develop very quickly.这就使得工 农业必须飞速发展。
四、构成强调句型。为了强调句子的某一成分，把引导词“it”用在句首，这种强调句 的结构是“It is （was ）＋所强调的成分（主语、宾语、状语）＋that…”，表达的意思为“是…， 正是…，就是…”。“it ”在这种句型中本身没什么实际意义。
My temperature! It's my back that hurts.我的体温！是我的背痛。
It is not only blind men who make such stupid mistakes. 决不只有盲人才犯这样愚蠢的错误。
这里需要说明的是：如被强调的是人，则后面用who或that，如被强调的是物或其他 情况，则用that。
It was yesterday that I met Bob in your room. 我就是昨天在你的房间遇见鲍勃的。
It was in your room that I met Bob yesterday. 我就是在你的房间昨天遇见鲍勃的。
It was Bob that I met in your room yesterday. 昨天我在你房间遇见的就是鲍勃。
来源于英语网
