从属连词that， whether等;
连接代词who，what，which等;
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
连接词
（1）从属连词∶that， whether等
1. that引导主语从句时只起引导作用，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whether及其他连词引导的主语从句放在句首、句后都可以。
如∶That she left him cut him to the heart.
Whether it will please them is not easy to say.
2. 连接代词∶who，what， which. whoever， whatever， whichever等
如∶What seems easy to some people seems difficult to others.
Which side will win is not clear.
连接副词∶when， where， how， why等
如∶Why he did it remains a mystery.
How he became a great scientist is known to us all.
位置
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
如∶
(1) It is a fact（a shame，a pity，good news….）that …
It is still uncertain whether he is coming or not.
It is certain that she will do well in her exam.
It is said that Mr. Green has arrived in Beijing.
（2）It+不及物*+主语从句
如∶It seemed （happened，doesn't matter，has turned out） that.
It happens that they were absent.
It doesn't matter whether she will come or not.
（3）It +及物*（被动语态）+主语从句
如∶It has been decided that the exhibition will not open on Sundays.