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一、名词性从句概念
在复合句中起词名词作用的句子叫名词性从句 (Noun Clauses）。 名词性从句的功能相当于名词，可分别作主句的主语、表语、宾语和同位语。因此，名词性从句可分为主语从句、表语从句、宾语从句和同位从句。
二、引导名词性从句的连接词
1．连接代词：who, whose, whom, what, which有词义，在从句中必须充当句子成分，如主语、表语、宾语或定语等。
2．连接副词：when, where, why, how有词义，在从句中必须充当成分，只能作状语。
3．连接词：that, whether, if, as if
1）that 无词义，在从句中不充当成分，做宾语时可省略；
2）if ，whether，as if虽有词义，但在从句中不担任成分。
4．连接代词：whatever, whoever, whomever, whichever
【注意】
1. 连接代词与连接副词在句中不再是疑问词，因而名词性从句中谓语不用疑问式，都必须与陈述句语序相同，即“主语＋谓语”。
2. 连接代词与连接副词有词义，在从句充当句子成分；连接词whether 和if（是否），as if（好象）有词义但在从句中不充当句子成分，只起连接作用；that无词义，在从句中不充当成分，做宾语时可省略。
三、名词性从句的用法
（一）主语从句：在复合句中用来作主语的句子叫做主语从句。
1．主语从句位于句首
That he finished writing the composition in such a short time surprised us all.
Whether we will go for an outing tomorrow remains unknown.
Whatever you did is right.
Who will be our monitor hasn't been decided yet.
What caused the accident remains unknown.
Which of the two is better remains to be seen.
When we will start the work is important.
2．为了避免句子头重脚轻，用it作形式主语，主语从句放在句末。
It doesn’t matter so much whether you will come or not.
It is true that he has made a very important discovery in chemistry.
It is a pity that we won't be able to go to the south to spend our summer vacation.
Is it likely that it will rain in the evening ?
3. 在主语从句中须注意：
1）主语从句一般用it作形式主语，放在句首，而将主语从句放到句末。如：
It is possible that he has stolen the car. 很可能他偷了小车。
Isn’t it strange that he should not have passed the test? 他测试没有通过，难道不奇怪吗？
2）if引导主语从句时，只能用it作形式主语，而将if引导的主语从句放到句末。如：
It is uncertain if he will leave for
3）that引导主语从句放在句首时，that不可省，但在句末口语中可省。如：
That he was ill yesterday is known now. 大家都已知道他昨天生了病。
It’s a pity (that) you missed the sports meeting last week.
4）连接代词引导主语从句只能放在句首。如；
Who has broken the glass is unknown. 谁打破了玻璃现在还不知道。
What he wants is all here. 他所要的东西都在这里。
5）在含有主语从句的复合句中，主句的谓语动词一般用第三人称单数形式，但what引导主语从句时，主句谓语动词的形式须根据句子的意思来判断。如：
How he worked it out is still a secret. 他是如何把它做出来的还是一个秘密
What they makes in this factory are TV sets. 他们在工厂里所做的东西就是电视机。
What we need is money. 我们需要的是钱。
（二）表语从句：在复合句中用来作表语的句子叫做表语从句。
The question was who could go there.
He has become what he wanted to be ten years ago.
She has remained where I stood yesterday for an hour.
His suggestion is that we should stay calm.
It seems as if it is going to rain.
The question is whether the enemy is marching towards us.
What she couldn’t understand was why fewer and fewer students showed interest in her lessons.
【注意】
1）连接词that一般不能省略，但当主句中含动词do的某种形式时，that可以省略。如：
What I want to do is (that) I can go up to him and thank him.
我想做的事是走到身边去感谢他。
2）表语从句中只能用whether，不能用if。
The question is whether they have so much money.
3）当句子的主语是：advice, demand, decision, desire, idea, insistence, instruction, order, plan, proposal, recommendation, request, requirement, suggestion, wish等名词时，表语从句须用虚拟语气，即：should +do。如：
My advice is that you should go to apologize to him. 我的建议是你应该去向他道歉。
（三）宾语从句：在复合句中用来作宾语的句子叫做宾语从句。如：
He asked what you were doing last night. 他问你昨天夜晚在干什么。
I didn’t know that he was wounded in the accident. 我原来不知道他在那事故中受了伤。
Can you explain why he didn’t come to the party last Sunday?
你能解释上周星期日他为什么没有来参加聚会吗？
【注意】
1）that 在口语中常被省略，但在正式的文体中，不能省；并且它一般不能引导介词的宾语从句，但可引导except, but, in 等少数介词的宾语从句。如：
I didn’t know (that) he was Li Lei. 我不知道他就是李蕾。
I will do anything I can to help you except that I’m ill.
只要我身体舒服，我愿做任何事情来帮助你。
2）宾语从句之后带有补足语成分时，一般须用it作形式宾语，把宾语从句放到句末，此时的that不能省。如：
The boy has made it clear that they can’t play with his toys.
那个男孩已清楚的表明他们不能玩弄他的玩具。
3）动词advise, ask, demand, desire, insist(坚决主张), order, propose, request, suggest(建议)等的宾语从句须用虚拟语气，即：用“(should) do”形式。
He asked that we (should) get there before nine o’clock. 他要求我们在九点钟之前赶到那儿。
The teacher advised us that we (should) not waste our time. 老师劝告我们不要浪费时间了。
4）宾语从句的时态与主句的时态须呼应。
① 当主句谓语动词的时态是现在时或将来时，从句的谓语动词可用任何所需要的时态表示。如：
He always says that he is our good friend. 他总是说他是我们的好朋友。
When the teacher knows what we have done, he will say that we have done a good deed.
当老师知道我们所干的事情时，他会说我们做了一件好事。
② 当主句谓语动词的时态为过去时，从句的谓语动词须用响相应的过去时。如：
He didn’t tell us he came from
He said he had read the book. 他说他读过这本书。
③ 但当从句的内容是客观真理或客观事实，而不受时间的限制时，即使主句谓语动词为过去时，从句仍用一般现在时。
Chairman Mao said that all the imperialists are paper tigers.
毛主席说一切帝国主义者都是纸老虎。
When we were children, we were told that
我们还是小孩子的时候，人们就告诉我们说中国在世界的东方。
5）whether与if都可以引导宾语从句，常可互换。但下面情况不能互换。
① 宾语从句是否定句时，只用if，不用whether。
I wonder if it doesn’t rain.
② 用if 会引起误解，就要用whether。
Please let me know whether you want to go.
（此句如果把whether改成if，容易当成条件句理解）
③ 宾语从句中的whether 与or not直接连用，就不能换成if；不直接连用，可换。
I don’t know whether or not the report is true.
I don’t know whether/ if the report is true or not.
④ 介词后的宾语从句要用whether引导；whether 可与不定式连用；whether也可引导主语从句、表语从句、同位语从句，还可引导让步状语从句，以上均不能换成if。
It depends on whether we have enough time.
They don’t know whether to go there.
（四）同位语从句：同位语从句在句中作某一名词的同位语，一般位于该名词（如：news, fact, idea, suggestion, promise等）之后，说明该名词的具体内容。这个从句就是同位语从句。如：
The news that
中国加入了世界贸易组织这一消息使我们大家兴奋不已。
He told us the truth that he fell down from his bike this afternoon.
他告诉了我们他从单车上摔下来这一真相。
【注意】
1）高中所学能带同位语从句的名词不多，常见的有：answer, belief, conclusion, decision, discovery, doubt, explanation, fact, hope, idea, information, knowledge, law, message, news, opinion, order, possibility, problem, promise, proof, proposal, question, report, risk, rumor, story, suggestion, truth, theory, thought, wish, word等。如：
He sent us a message that he won’t come next week. 他送信来告诉我们说，下周他不来了。
There is no doubt that he is fit for this job. 毫无疑问，它适合这一工作。
2）表示“命令、建议、要求” advice, demand, decision, desire, idea, insistence, instruction, order, plan, proposal, recommendation, request, requirement, suggestion, wish 等名词的同位语，须用虚拟语气。
He made clear his desire that he should work hard to go to college.
他已清楚的表明了他要努力学习考上大学这一愿望。
Soon came the order that all the people should obey the rules.
不久命令下来了，所有的人不得违犯制度。
3）连词that引导同位语从句，that不能省，且在从句中没有任何意义，不充当任何成分。
The idea that you can do this work well without thinking is quite wrong.
4）连词whether引导同位语从句，不能用if。
The question whether we should call in a specialist was answered by the family doctor.
5）连接代词what, who, whom, whose, which引导同位语从句
I have no idea what size shoes she wears.
I have no idea which wine is best—it’s a matter of personal taste.
The question who will take his place is still not clear.
6）连接副词when, where, how, why引导同位语从句
We haven’t yet settled the question where we are going to spend our summer vacation.
I have no idea when they will come .
7）同位语从句与定语从句的区别：
同位语从句与定语从句形相似而实质不同，具体表现在以下几个方面：
⑴ 被修饰的词不同。同位语从句的前面一般是数量有限的、表示抽象意义的，能够包含内容的名词；而定语从句的先行词是无数的普通的指人或物的名词。
⑵ 从句的作用不同。同位语从句表示同位名词的具体内容；而定语从句是对先行词起修饰、限制作用。
⑶ 引导词的作用不同。引导同位语从句的that是连词，在从句中不担任任何成分，但不能省略；而引导定语从句的that为关系代词，在从句中担任句子的某种成分，当担任定语从句中的宾语时，that可以省略。
⑷ 引导词不同。定语从句不能用what, how, whether引导；而同位从句可以用what, how, whether引导。
⑸ 判定定语从句和同位语从句的方法。同位语从句可充当同位名词的表语；而定语从句不能充当先行词的表语。
