名词性从句是由if, whether, that 和各种疑问词充当连接词所引导的从句，其功同名词一样。
一．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。
1. it 作形式主语和it引导强调句的
it is a pity that you didn't go to see the film.
it doesn't interest me whether you succeed or not.
it is in the morning that the murder took place.
it is john that broke the window.
