主语从句 定义：在句子中担当主语的是一个从句，这个从句就叫做主语从句。
主语从句是在复合句中充当主语的从句，通常放在主句谓语动词之前或由形式主语it代替，而本身放在句子末尾。
一、It 作形式主语和it引导强调句的比较
1.主语从句常用it作形式主语，一般常用句型为：It is+{名词/形容词/过去分词}+主语从句
例如：It is still unknown which team will win the match.
另外，还有一些比较多见的结构：
It turned out that……;
It has been proved that……;
It happened/occurred that……;
It is well-known that……等等
2.而强调句则不同，它的结构是：It+be+被强调部分+that+从句
强调主语：It is the times that produce their heroes.时势造英雄
强调宾语：It is English that Prof.Lin teaches us.
强调状语：It was in shanghai that I saw the film.
二、用it 作形式主语的结构
1.It is +名词+that从句
It is common knowledge that …是常识
2.it is +形容词+that从句
It is strange that… 奇怪的是……
3.it +不及物动词+that从句
It happened that… 碰巧……
4.it is+过去分词+that从句
It has been proved that… 已证实……
