在主从复合句中充当主语、宾语、表语和同位语的从句分别称为主语从句、宾语从句、表语从句和同位
语从句。这些从句在句中所起的作用相当于名词在句中所起的作用，因此统称为名词性从句。引导名词性
从句的引导词有：
that引导宾语从句时常常可以省略，但引导其他名词性从句时一般不能省略；whether和if引导名词性从句时均不能省略。
I know (that) I rely too much on the computer. 我知道自己过于依赖电脑。
I asked him whether/if he had done this thing by himself.我问他这件事是否是他自己做的。连接词whether可以引导所有名词性从句，而if主要引导动词后的宾语从句和不在句首的主语从句；whether引导的从句可以放在句首，而if引导的从句一般只能放在动词之后；whether可与or (not)连用，f一般不与or (not)连用；引导动词后的宾语从句时，if和whether可以互换使用。
名词性从句一律用陈述句语序。如：
Whether he can come to the party on time depends on the traffic.
他能否准时来参加聚会取决于交通状况。
None of us knows where this kind of books can be bought.
我们中谁也不知道在哪里可以买到这种书。
The real measure of success is how happy you are.
成功的真正标准是你的幸福程度。
The thought that she might never see him again made her eyes fill with tears.一想到可能再也见不到他，她就热泪盈眶。
1.主语从句
(1)主语从句是在复合句中作主语的名词性从句，通常位于主句谓语动词之前。主语从句通常由连接词d，whether,f和连接代词 whet,who, which,whatever, whoever 等以及连接副词 how, when, whereh引导。连接词 hat在从句中不充当任何语法成分，只起连接作用且无词义，但一般不能省略；连接代词和连接明词在句中既保留自己的含义，又起连接作用，并在从句中充当一定的语法成分。
That the famous painter will hold an exhibition is certain.
那位著名画家要举办一场展览会是确定的。
Whether the project should be carried out is still a problem.
这项工程是否应该实施仍然是一个问题。
What he feels hard is to master German in two months.
他认为困难的是要在两个月内掌握德语。
When we should go to Lijiang for vacation remains to be discussed.
我们应当什么时候去丽江度假仍有待讨论。
(2)在含有主语从句的复合句中，常用t作形式主语，并将主语从句置于句末。常见的结构有：① lt+be 动词+过去分词/形容词/名词短语+主语从句。
It is reported that the temperature will decline sharply in the near future.
据报道，在不久的将来气温将大幅下降。
II was quite clear that he made you angry deliberately.
十分明显，他故意让你生气。
It is common knoneledge that Mount Qomolangma is the world's highest peak.
珠穆朗玛峰是世界第一高峰是常识。
② 1t+不及物动词+主语从句。
It doesn't matter whether he has lied or not.
他是否说谎并不重要。
It suddenly occurred to me that I had made a big mistake.
我突然想到自己犯了一个大错。
③ t+及物动词+宾语+主语从句。
It makes no difference who you are.
你是谁并没有影响。
It struck me that we could hire new employees to help us.
我突然想到我们可以雇用新员工来帮我们。
④t+be动词+介词短语+主语从句。
It is of great importance that we improve our own professional skills.
我们提升自己的专业技能是极为重要的。
2.宾语从句
宾语从句是在复合句中作宾语的名词性从句，通常位于动词(及物动词)或介词之后。宾语从句可以由连接代词、连接副词或连接词that/whether/if引导。另外，that引导宾语从句时常常可以省略，但当一个句子中有多个并列的宾语从句时，第一个从句中的that可以省略，其后从句中的that往往不可以省略。
I'll try to make up for whar you have lost because of this.
我将尽力弥补你为此而失去的东西。
Tell me tcho/whom you associate with, and I will tell who you are.告诉我你和谁交往，我将看出你是什么样的人。(观其友，知其人。)I wonder why this television set didn't work.
我想知道这台电视机为什么不运转了。
He said (that) we were too young to understand the matter and that we should turn to our professor.
他说我们太年轻，还无法理解这件事，而且我们应该向我们的教授寻求指教I asked John whether/if someone had helped him with the housework.我问约翰是否有人帮他做了家务。
形容词sure, afraid,certain,sorry等作表语时，其后可以接that引导的宾语从句，that常可省略；也可以用连接代词、连接副词或whether引导从句。如：
We are sure (that) it will succeed.我们肯定它会成功。
1动词find,feel, think, consider, make, believe, suppose等后接宾语补足语时，常用it作形式宾语，而将宾语从句后置。如：
We have made it clear that our purpose is to learn knowledge.
我们已经清楚地表明我们的目的是学习知识。
②动词doubt后面接宾语从句时，在肯定句中，一般接whether或if引导的宾语从句；在否定句和疑问句中，常接that引导的宾语从句。如：
I doubt whether/if they can swim across the river.
我不敢肯定他们能否游过河去。
She never doubted that she was right about the thing.她从未怀疑过自己在这件事上是正确的。Do you doubt that she can compete in the match?你认为她不一定能参加比赛吗？
3.表语从句
表语从句是在复合句中作表语的名词性从句，通常位于连系动词之后，其后可接表语从句的连系动词有：be, look, remain,seem等。表语从句的引导词与主语从句的引导词大致一样。使用表语从句的基本结构为：主语+连系动词+表语从句。
Our plan is that we'll go there once a week.
我们的计划是我们每周将会去那里一次。
The problem is whether the new policy can be put into practice.问题是新的政策能否被付诸实践。
Personal fame and fortune is what they're going after at present.他们目前正在追求的是个人名利。
The question is how we can find an effective way to store the sun's heat.问题是我们如何能找到一种有效的方法来储存太阳的热量。
当主语为reason，后面的表语从句表示原因时要用that引导，而不用why或because，构成句型The reason (why..) is/was that...意为“(······的)原因是……”。如：
The reason why he did not join us was that he had to attend a lecture.他没有加入我们的原因是他得参加一个讲座。
表语从句还可以用as if，as though, like, because引导，其常用结构为：1 It looks/seems...+as if/as though/like...。如：
It looks/seems as if/as though/like you have got no experience.看来你似乎没有经验。
2 ItThis/That is/was/may be because...。如：
He slept in class. That may be because he didn't sleep well last night.他在课堂上睡觉了。那可能是因为他昨晚没有睡好。
4. 同位语从句
同位语从句是在复合句中作同位语的名词性从句。同位语从句用于解释说明其所修饰的名词(一般是描象名词)的具体内容。同位语从句可以由连接词that,whether引导，也可以由连接代词what,which,oho等D及连接副词whon,where, why,hom引导。其中 thot 在同位语从句中不充当任何语法成分，只起连接作用且无词义，但不能省略。
常见的后面可跟同位语从句的名词有：edvice, conclusion, chance, demand, donbt, cvidernee, faet, hope,idea infomaton, mesage, eas, onder,podlon, proonise, qiestion, fegvest, momr,leetion, thought, tmuth等。
There is only a fifty-fifty chance that we will win the game.
我们会赢得比赛的机会只占一半。
There is still some hope that we'll find a solution to this problem
我们还有些希望找到这个问题的解决办法。
The idea that the number“4" brings bad luck seems to be quite absurd.
认为数字4会带来坏运气的这一看法似乎很荒唐。
He made the suggestion that they go for a drive.
他建议他们开车去兜兜风。
The students asked me the question whether the book was worth reading学生们问我这本书是否值得阅读这个问题。
The question who will be their new manager is still not clear.
谁会是他们的新经理这个问题尚不清楚。
同位语从句有时可以不紧跟在其所解释说明的名词后面，而是被其他的内容隔开。如：
A thought suddenly occurred to me thatwe might have done the thing wrong.我突然想到我们可能做错了这件事。