在英语中， it 有许多不同的用法。它既可以用作代词(如人称代词、非人称代词)，也可以用作引导词(作形式主语或形式宾语)，还可以用来构成强调句型。
1. it 用作人称代词，指代前面提到过的人或事物。如：
He bought a dictionary and gave it to me. (指代物品 dictionary )
John hates children， and it is difficult to say why.
(指代前面提到的事情)
2. 表示天气、时间、温度、距离等。如：
It was raining when I left the office.
(表示天气)
It's an hour's walk from my home to the school.
(表示距离)
It's a long time since we met last time. (表示时间)
3. it 用作形式主语或形式宾语
根据句子结构的需要，人们常用 it 作形式主语或形式宾语，而把真正的主语或宾语(不定式短语、动词- ing 形式短语或名词性从句)置于句尾。如：
It is important to learn English well.
It is a pity that I didn't think of it earlier.
We found it strange that no one would take the money.
4. it 用在强调句型中
英语中，有时为了突出句子中的某一成分，达到强调或使人特别注意该成分的目的，人们常用“ It is / was … that/ who …”句型把整个句子分割成前后两个部分，使之各自有自己的谓语动词，中间由 that 或 who 连接起来成为一个新的句子，人们习惯上称这一句型为强调句型。如：
Mathilde wore the beautiful diamond necklace to attend the palace ball with her husband last night.
→ It was Mathilde who wore the beautiful diamond necklace to attend the palace ball with her husband last night. (强调主语)
→ It was the beautiful diamond necklace that Mathilde wore to attend the palace ball with her husband last night. (强调宾语)
→ It was with her husband that Mathilde wore the beautiful diamond necklace to attend the palace ball last night. (强调方式状语)
→ It was last night that Mathilde wore the beautiful diamond necklace to attend the palace ball with her husband. (强调时间状语)
另外，学习 it 的用法时，还要注意学会使用下列常用句式：
It takes / took / will take sb some time to do sth
做某事花费某人多少时间
It looks / looked as if …看起来好像……
It seems / seemed that …似乎……
It so happened that …碰巧……
It turns / turned out that …原来是(结果是)……
It is said / reported / believed / thought / hoped / well-known / suggested that …据说 / 报道 / 相信 / 认为 / 希望 / 众所周知 / 建议……
It occurred to sb that …某人想起……
It will be / was + some time before …要过多久才 / 过了多久才……
It is / was / has been / had been + some time + since …自从……已经有多久时间了……
