1.可接表语从句的连系动词有be, look, seem, sound, appear等。
China is no longer what she used to be.
The question remains whether they will be able to help us.
At that time, it seemed as if I couldn't think of the right word.
2. 从属连词whether, as, as if / though引导的表语从句。
He looked just as he had looked ten years before.
It sounds as if someone is knocking at the door.
3. because, why引导的表语从句。
That's because he didn't understand me.
That's why he got angry with me.
The reason why I was sad was that he didn't understand me.
4. 连接代词who, whom, whose, what, which, whoever, whatever, whichever
连接副词 where, when, how, why
The problem is who we can get to replace her.
The question is how he did it.
That was what she did this morning on reaching the attic.
The trouble is that I have lost his address.
在表示建议、劝告、命令含义的名词后的表语从句，谓语动词需用“should+动词原形” 表示，should可省略。常见的词有：advice, suggestion, order, proposal, plan, idea等。
My suggestion is that we (should) start early tomorrow.