名词性从句的用法
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三类：连词：that（无任何词意）；whether, if（均表示“是否”表明从句内容的不确定性）；as if ,as though（均表示“好像”，“似乎”）。
以上在从句中均不充当任何成分
连接代词：what, whatever, who, whoever, whom, whose, which. Whichever, whomever
连接副词：when, where, how, why
不可省略的连词：
1. 介词后的连词
2. 引导主语从句和同位语从句的连词不可省略。
That she was chosen made us very happy.
We heard the news that our team had won.
比较：
Whether与if 均为"是否"的意思。 但在下列情况下，whether 不能被if 取代：
1. Whether引导主语从句并在句首；
2. 引导表语从句
3. Whether从句作介词宾语；
4. 从句后有"or not"
Whether he will come is not clear.
大部分连接词引导的主语从句都可以置于句末，用 it充当形式主语。
It is not important who will go.
It is still unknown which team will win the match.
二、具体分类
1．主语从句
作句子主语的从句叫主语从句。主语从句通常由从属连词that，whether，if和连接代词what，who，which，whatever，whoever以及连接副词how，when，where，why等词引导。that在句中无词义，只起连接作用；连接代词和连接副词在句中既保留自己的疑问含义、又起连接作用，在从句中充当从句的成分。例如：
What he wants to tell us is not clear. 他要跟我们说什么，还不清楚。
Who will win the match is still unknown. 谁能赢得这场比赛还不得而知。
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。
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。
【典例1】(2009·上海卷)It is immediately clear ____ the financial crisis will soon be over.
A. since B. what C. when D. whether
【答案】D
【解析】考查主语从句的用法。该句的意思时：经济危机是否会很快结束是很明显的事情。“是否”用whether表示，不能选when是因为从句中由soon这一实践状语。
【典例2】(2008·山东卷)_____ was most important to her, she told me, was her family.
A. It B. This C. What D. As
【答案】C
【解析】she told me是插入语，可删除。what引导主语从句，在从句中充当主语。
有时为避免句子头重脚轻，常用形式主语it代替主语从句作形式主语放于句首，而把主语从句置于句末。主语从句后的谓语动词一般用单数形式。常用句型如下：
（1）It + be + 名词 + that从句
（2）It + be + 形容词 + that从句
（3）It + be + 动词的过去分词 + that从句
（4）It + 不及物动词 + that 从句
另注意在主语从句中用来表示惊奇、不相信、惋惜、理应如此等语气时，谓语动词要用虚拟语气“(should) +do”，常用的句型有：
It is necessary (important, natural, strange, etc.) that …
It is a pity (a shame, no wonder, etc.) that…
It is suggested (requested, proposed, desired, etc.) that…
【典例】(2009·天津卷)It is obvious to the students ______ they should get well prepared for their future.
A. as B. which C. whether D. that
【答案】D
【解析】考查名词性从句。本空格处应是一个主语从句的引导词，因该主语从句不缺成分，且句意完整，故应用不作任何成分的连词that。
2．宾语从句
名词句用作宾语的从句叫宾语从句。引导宾语从句的关联词与引导主语从句表语从句的关联词大致一样，在句中可以作谓语动词或介词及非谓语动词的宾语。
（1） 由连接词that引导的宾语从句
由连接词that引导宾语从句时，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。例如：
He has told me that he will go to Shanghai tomorrow. 他已经告诉我他明天要去上海。
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。
注意：在demand，order，suggest，decide，insist，desire，request，command等表示要求、命令、建议、决定等意义的动词后，宾语从句常用“（should）+ 动词原形”。例如：
I insist that she (should) do her work alone. 我坚持要她自己工作。
The commander ordered that troops (should) set off at once. 司令员命令部队马上出发。
（2）用who，whom, which, whose, what, when, where, why, how, whoever, whatever, whichever等关联词引导的宾语从句相当于特殊疑问句，应注意句子语序要用陈述语序。例如：
I want to know what he has told you. 我想知道他告诉了你什么。
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。
【典例1】(2008·北京卷)The companies are working together to create _______ they hope will be the best means of transport in the 21st century.
A. which B. that C. what D. who
【答案】C
【解析】create后为宾语从句，从句中they hope是插入语，可删除。因为从句中缺少主语，所以用what引导该从句。
【典例2】(2009· 全国卷Ⅰ) Could I speak to is in charge of International Sales please?
A. who B. what C. whoever D. whatever
【答案】C
【解析】 考查名词性从句。题干中介词to后面为宾语从句，从句中缺少主语，因此填whoever。
（3）用whether或if引导的宾语从句，其主语和谓语的顺序也不能颠倒，仍保持陈述句语序。此外，whether与if 在作“是否”的意思讲时在下列情况下一般只能用whether，不用if：
a. 引导主语从句并在句首时；b. 引导表语从句时；c . 引导从句作介词宾语时；d. 从句后有“or not”时；e. 后接动词不定式时。例如：
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。
The question is whether she should have a low opinion of the test?
Everything depends on whether we have enough money. 一切要看我们是否有足够的钱。
I wonder whether he will come or not. 我想知道他来还是不来。
Can you tell me whether to go or to stay? 你能否告诉我是去还是留？
（4）注意宾语从句中的时态呼应，当主句动词是现在时，从句根据自身的句子情况，而使用
不同时态。例如：
he studies English every day. （从句用一般现在时）
he studied English last term. （从句用一般过去时）
I know (that) he will study English next year. （从句用一般将来时）
he has studied English since 1998. （从句用现在完成时）
当主句动词是过去时态（could, would除外），从句则要用相应的过去时态，如一般过去时，过去进行时，过去将来时等；当从句表示的是客观真理，科学原理，自然现象，则从句仍用现在时态。例如：
The teacher told us that Tom had left us for America．
（5）think, believe, imagine, suppose等等动词引起的否定性宾语从句中，要把上述主句中的动词变为否定式。即将从句中的否定形式移到主句中。例如：
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。
3．表语从句
在句中作表语的从句叫表语从句。引导表语从句的关联词与引导主语从句的关联词大致一样，表语从句位于连系动词后，有时用as if引导。其基本结构为：主语 + 系动词 + that从句。例如：
The fact is that we have lost the game. 事实是我们已经输了这场比赛。
That's just what I want. 这正是我想要的。
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。
It looks as if it is going to rain. 看上去天要下雨了。
需要注意的，当主语是reason时，表语从句要用that引导而不是because。例如：
The reason why he was late was that he missed the train by one minute this morning .
【点拨】whether 可引导表语从句，但与之同义的if却通常不用于引导表语从句。
【典例1】 (2009·山东卷)The little girl who got lost decided to remain ________she was and wait tor her mother.
A. where B. what C. how D. who
【答案】A
【解析】remain是系动词，后加表语从句，由句意可知，从句缺地点状语，所以选A。
【典例2】(2008·天津卷)The last time we had grat fun was _______ we were visiting the Water Park.
A. where B. how C. when D. why
【答案】C
【解析】was后为表语从句，此处when与the last time相呼应，根据句意可排除其他选项。
4． 同位语从句
同位语从句说明其前面的名词的具体内容。同位语从句通常由that引导，可用于同位语从句的名词有advice，demand，doubt，fact，hope，idea，information，message，news，order，problem，promise，question，request，suggestion，truth，wish，word等。例如：
The news that we won the game is exciting. 我们赢得这场比赛的消息令人激动。
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。
【典例】(2009· 重庆卷) We should consider the students' request ________ the school library provide more books on popular science.
A. that B. when C. which D. where
【答案】A
【解析】考查名词性从句。根据题意知，空格处是同位语从句的引导词，修饰中心名词request，且在句子中不充当成分，故用that。这句话的意思是我们应该考虑学生的这个要求，即学校图书馆应该多提供一些大众科学方面的书籍。
三、对比与用法
1．同位语从句和定语从句的区别：
that作为关系代词，可以引导定语从句，充当句子成分，在从句中作宾语时可以省略； that引导同位语从句时，起连词的作用，没有实际意义，不充当句子成分，一般不能省略。
试比较下面两个例句:
I had no idea that you were here．（that引导同位语从句，不能省略）
Have you got the idea（that）this book gives you of life in ancient Greece？（that引导定语从句，作宾语，可以省略）
2．that-从句作主语通常用it作先行词，而将that-从句置于句末。
用it作形式主语的that-从句有以下四种不同的搭配关系：
a. It + be +形容词+ that-从句
It is necessary that… 有必要……
It is important that… 重要的是……
It is obvious that… 很明显……
b. It + be + -ed 分词+ that-从句
It is believed that… 人们相信……
It is known to all that… 从所周知……
It has been decided that… 已决定……
c. It + be +名词+ that-从句
It is common knowledge that… ……是常识
It is a surprise that… 令人惊奇的是……
It is a fact that… 事实是……
d. It +不及物动词+ that-分句
It appears that… 似乎……
It happens that… 碰巧……
It occurred to me that… 我突然想起……
It is quite clear that the whole project is doomed to failure.
很清楚，整个计划注定要失败。
It's a pity that you should have to leave.
你非走不可真是件憾事。
3．否定转移
（1） 将think, believe, suppose, expect, fancy, imagine等动词后面宾语从句的否定词转移到主句中，即主句的谓语动词用否定式，而从句的谓语动词用肯定式。
I don't think I know you. 我想我并不认识你。
I don' t believe he will come. 我相信他不回来。
注意：若谓语动词为hope,宾语从句中的否定词不能转移。
I hope you weren't ill. 我想你没有生病吧。
（2）将seem, appear 等后的从句的否定转移到前面。
It doesn't seem that they know where to go.
看来他们不知道往哪去。
It doesn't appear that we'll have a sunny day tomorrow.
看来我们明天不会碰上好天气。