主语从句、表语从句和宾语从句在复合句中的功能和名词相同，分别作主语、表语和宾语，因此，这三种从句可合称为名词性从句，具有共同特征：

1）关联词相同
连词： that, whether, if
疑问代词：who, what, which, whoever, whatever, whichever
疑问副词：when, where, how, why, whenever, wherever, however

2）三种从句均不能用逗号和主句分开

1．主语从句（subject clause）：在句中作主语，位置与陈述句基本结构中的主语相同。

That he will join in the discussion is certain.
Who stole this dictionary is unknown.
Whoever arrives first will be given the prize.
How the boy has won the first prize is interesting to us all.
What the woman said surprised everyone.
* 主语从句放在句首时，句子显得笨重，因此常把that引导的从句放在句末，用it作形式主语.
It is certain that he will join in the discussion.

2. 表语从句（predictive clause）：在句中作表语，位于主句中的连系动词之后。

Our rule is that everyone is to come back before midnight.
This is what I want to get.
That is why she is so sad.
The question is whether this job can be finished before 5 o'clock.

3. 宾语从句（object clause）：作谓语动词的宾语；也可作介词、非限定动词（动词不定式、动名词、分词）和某些形容词的宾语。

1) 由连词that引导的宾语从句
We believe that the earth goes around the sun.
I am sure that Joan can be chosen for this position.
Seeing that it was going to rain, we decided to work inside the house.

2）由连词whether或if引导的宾语从句
I want to know whether you are still interested in making friends with me.
I wonder if there is an interesting film on TV tonight.

3）由疑问代词和疑问副词引导的宾语从句
Do you know when we are going to the sports center?
Does your neighbor know who has stolen your newspaper?
I am wondering what my husband is doing now.
Does anyone know why such an important meeting should be put off till tomorrow?
有人知道为什么如此重要的会议要推迟到明天吗？
I am very surprised at what he said, and even more surprised at what he did.
This weekly schedule can make you aware of how you spend your time.
I will tell the news to whoever comes first.
* 宾语从句有时太长，因此往往用it作形式宾语，而真正的宾语则放在其后。
I think it improper that he will be chosen as the candidate for the champion.
我认为他被选为竞选候选人是不妥的。
I cannot believe it that someone has stolen the necklace from the safe.
有人竟从保险箱里偷走了项链。