1、引导主语从句的连接词有从属连词that, weather, 连接代词who, what, which等等，咒邃狁廖连接副词when, where, how, why等。主语从句在句首时，必须由连接词引导，不能省略这些连接词
2、that引导主语只起引导作用，本身无实际意义，在主语从句中不充当任何成分，但不能省略，由weather及其他连词引导的主语从句放在句首、句后都可以
3、连接代词引导的主语从句，例如what seems easy to some people seems difficult to others, which side will win is not clear
4、连接副词引导的主语从句，例如why he did it remains a mystery. how he become a great scientist is known to us all
5、主语从句可以前置，也可以后置，用it作形式主语时，须把主语从句放在句末，常用下面几种句型：it+be+表语（名词，形容词，过去分词）+主语从句
6、也包括it+不及物动词+主语从句以及it+及物动词（被动语态）+主语从句等句型，例如it has been decided that the exhibition will not open on sundays.
