根据it的用法特点可归纳为20个句型
1. It is + 被强调部分 + that …
该句型是强调句型。将被强调的部分放在前面，其它部分置于that之后。被强调部分可以是主语，宾语，表语 或状语。强调的主语如果是人，that可以由who换用。如果把这种句型结构划掉后，应该是一个完整无缺的句子。这也是判断强调句型与其它从句的方法。
It was about 600 years ago that the first clock with a face and an hour hand was made.
It was they that （who ） cleaned the classroom yesterday.
It was in the street that I met her father.
2. It is not until + 被强调部分 + that …
该句型也是强调句型。主要用于强凋时间状语，译成汉语“直到…才…”，可以说是 not … until … 的强调形式。
It was not until she took off her dark glasses that I realized she was a famous film star.
= Not until she took off her dark glasses did I realize she was a famous film star.
= I didn't realize she was a famous film star until she took off her dark glasses.
3. It is clear （ obvious, true, possible, certain…） that ……
该句型中it 是形式主语，真正的主语是that 引导的主语从句，常译为“清楚（显然，真的，肯定…）” 是主语从句最常见的一种结构。
It is very clear that he's round and tall like a tree.
= That he's round and tall like a tree is very clear.
4. It is important （ necessary, right, strange, natural…） that …
该句型和上一个同属一个句型。由于主句中的形容词不同，that 后的从句中要用虚拟语气（should + 动词原形），should 可以省去。建议记住该句型中的形容词。
It is important that we （should） learn English well.
It is necessary that he （should） remember these words.
5. It is said （reported, learned…） that …
该句型中的it 仍是形式主语，真正主语是that 引导的主语从句。该结构常译为“据说（据报道，据悉…）”。
It is said that he has come to Beijing.
It is reported that another earth satellite has been put into orbit.
6. It is suggested （ ordered … ） that …
该句型和上一个同属一个句型。主句中的过去分词是表示请求，建议，命令等词时，that 后的从句要用虚拟语气（should + 动词原形），should 可以省。常译为“据建议；有命令…）
It is suggested that the meeting （ should ） be put off.
It was ordered that we （ should ） arrive there in two hours.
7. It is a pity （ a shame … ） that …
该句型中，that后的从句一般用虚拟语气（should + 动词原形），should可省去。表示出乎意料，常译为”竟然“。没有这种意义时，则不用虚拟语气。
It is a pity that such a thing （ should ） happen in your class.这种事竟然发生在你们班上，真是遗憾！
It is a pity that he is ill. 他生病了，真遗憾！