it做形式主语和形式宾语的用法句型
1.It is/was + 被强调部分 + that/who ... 该句型是强调句型。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。
一般陈述句：Tom bought a book in this bookshop yesterday.
It is Tom who/that bought a book in this bookshop yesterday. 强调主语
It is a book that Tom bought in this bookshop yesterday. 强调宾语
It is in this bookshop that Tom bought a book yesterday. 强调地点状语
It is yesterday that Tom bought a book in this bookshop. 强调时间状语
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. 强调句
= Not until she took off her dark glasses did I realize she was a famous film star. 倒装句
= I didn't realize she was a famous film star until she took off her dark glasses. 一般句式
3. It is+形容词（clear /obvious/true/possible/certain…）+ that 从句 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+形容词（important /necessary/ right/strange/natural...）+that 从句(should) do 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。
It is necessary that we (should)learn a foreign language.
