在句子中起名词作用的句子叫名词从句 (Noun Clauses)。 名词从句的功能相当于名词词组，它在复合句中能担任主语、宾语、表语、同位语、介词宾语等，因此根据它在句中不同的语法功能，名词从句又可分别称为主语从句、宾语从句、表语从句和同位语从句。

一、引导名词性从句的连接词
　　引导名词性从句的连接词可分为两类：
1．名词性that从句，即以that为连接词的名词性从句。that只能起连接主句和从句的作用，不充当从句的任何成分，无词义。
2．名词性wh-从句，即以wh-词连接的名词性从句。Wh-词包括：
　　 连接代词：who, whom, whose, what, which, whoever, whatever, whichever
　　连接副词：when, where, how, why
　　连接词whether, if不充当句子的任何成分。

二、主要从句类型

1．主语从句
连词
that
 That he will come and help you is certain.
他来帮助你是确实无疑的。		 that在句首不可省去
wheter
 Whether there is life on the moon is an interesting question.
月球上有没有生命是个有趣的问题。		 主语从句中只能用whether不可用if
连
接
代
词
who
what
which
whatever

when
where
why
how

 Who will win the match is still unknown.
谁能赢得这场比赛还不得而知。
What he wants to tell us is not clear.
他要跟我们说什么，还不清楚。
Where the English evening will be held has not yet been announced.
英语晚会将在哪里举行，还没有宣布。
It is known to us how he became a writer.
我们都知道他是如何成为一名作家的。		 主语从句放在句首，句子常显得笨重，因此一般把它移到句子后面，前面用引导词“it”来作形式主语。
2．宾语从句：
陈述意义
that

I believe(that) he is honest.
我相信他是忠诚的。
We must never think(that) we are good in everything while others are good in nothing.
我们决不能认为自己什么都好，别人什么都不好。

 that在句中不担任任何成分，在口语或非正式的文体中常被省去，但如从句是并列句时，第二个分句前的that不可省。
疑问意义
if
whether

I wonder whether he will come or not.
我想知道他来还是不来。
Everything depends on whether we have enough money.
一切要看我们是否有足够的钱。
I don't know if(whether) it is interesting.
我不知道它是否有意思。
He doesn't care if it isn't a fine day.
他不在乎天气是否好。

whether常与or not连用，不能用if代替。
作介词宾语要用whether不能用if。
从句是否定句时一般用if引导。
特殊疑问意义 who, whom, which, whose, what, when, where, why, how, whoever, whatever, whichever

Please tell me what you want.
请告诉我你需要什么？
She always thinks of how she can work well.
她总是在想怎样能把工作做好。
She will give whoever needs help a warm support.
凡需要帮助的人，她都会给予热情的支持。

 宾语从句作及物动词宾语也可作介词的宾语。
  We must make it clear that anyone who breaks the law will be punished.
我们必须认清无论谁违反了法律都要受到惩罚。		 如果宾语从句后面有宾语补足语，则用it作形式宾语，将从句后置。
 

We don't think you are here.
我们认为你不在这。
I don't believe he will do so.
我相信他不会这样做。

 think, believe, imagine, suppose等动词引出的宾语从句，要将从句中的否定形式，移动主句中。
3．表语从句：
连词

that
whether
as if

The problem is(that) they can't get here early enough.
问题是他们不能很早到达这里。
It looks as if it's going to rain.
看起来天要下雨。

 在非正式的文体中that可以省去。
连
接
代
词

who
what
which

That's just what I want.
这正是我想要的。
The question is who(which of you) will be the next speaker.
问题是谁（你们哪一位）接着发言。

 表语从句位于主句系动词之后
连接副词

when
where
why
how

This is where our problem lies.
这就是我们的问题所在。
That is why he didn't come to the meeting.
那就是他为什么不到会的原因。

  
4．同位语从句
由连词that引导，不担任成分，也可有when, how, where等引导。

The news that he had landed on the moon spread all over the world. 他曾在月球上登陆这个消息传遍世界。
I have no idea when he will come back home.
我不知道他什么时候回来。
The thought came to him that Mary had probably fallen ill.
他想到可能玛丽生病了。
He must answer the question whether he agrees to it or not.
他必须回答他是否同意此事这样一个问题。

 同位语从句说明其前面的名词的具体内容，常用的名词如：fact, news, idea, hope, thought, question, order, fear, doubt, word, proof, belief, story等。