什么是主语从句

顾名思义就是主语是一句有引导词的句子
What he wants is a book.
How he was successful is still a puzzle.
Why he has come to New York is a story of struggle and success.
多数情况由it作形式主语，而把主语从句放在主语之后。例如：
It is reported that no one was killed in the earthquake. 据报道地震中没有人死亡。

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。
例如：It is a fact that … 事实是…