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。
It 作形式主语和it引导强调句的区别
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 a question whether she will come or not.
It is strange that you should like him.
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 teaches us.
强调状语：It was in shanghai that I saw the film.
