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所谓名词从句，就是把完整句子当作名词来使用，在另一个句子中充当某种成分。一般来讲，名词在句中主要充当四种成分：主语、宾语、表语和同位语。同样，我们可以把一个完整的句子当作名词来使用，也在另一个句中充当这四种成分，于是便有了常说的四种名词从句：主语从句、宾语从句、表语从句和同位语从句。
现在问题的关键是：我们可以把何种句子当作名词来用呢？
经研究发现，能够相当于名词作用的句子主要有三类：陈述句、一般疑问句以及特殊疑问句。因此，名词从句可简单概括为这样一句话：名词从句就是用三种句子来分别充当另外一个句子的四种句子成分。这即是名词从句的本质特征。
在《英语语法新思维中级教程——通悟语法》里，我们具体讨论了陈述句、一般疑问句和特殊疑问句是如何按照一系列规则进行变化后才能充当四种句子成分的，在此不再赘述。下列表格给出了名词从句的引导词与简单句之间的对应关系。请注意这些名词从句的连接词的用法特点。
从上面的表格中我们有以下重要发现，这是要求读者认真理解和掌握的：
第一，对于陈述句，连接词 that 是后来添加上去的，此时的 that 自然不充当从句里的任何成分，而且也没有任何意义，只起连接主句和从句的作用。
第二，对于一般疑问句，连接词 whether/if 也是后来添加上去的，目的是为了保留句子的疑问意义，有“是否”的含义，即 whether/if 是有意义的，这不同于 that 。与 that 相同的是， whether/if 也不充当任何句子成分。
第三，对于特殊疑问句，我们不需要添加任何连接词，因为特殊疑问词就充当了连词，从而变成了相应的连接词。但关键是，我们需要把倒装语序还原成陈述语序。这些特殊疑问词都是在句中充当各种成分的，比如连接代词可以充当主语、宾语或表语，而连接副词则充当状语。
需要提醒读者注意的是，我们要特别关注这些连词是否在句中充当句子成分。比如： that 不作任何句子成分，只起连接作用，后面要接陈述句； what 一定充当从句的主语或宾语等成分； which 和 whose 后面都要接名词等等。这些都是考试重点，比如下面这道考研真题：
Concerns were raised witnesses might be encouraged to exaggerate their stories in court to ensure guilty verdicts.
A. what
B. when
C. which
D. that
正确答案 D 。
妙语点睛 本题考查的是名词从句中连词的使用。我们看到从句 witnesses might be encouraged to exaggerate their stories in court to ensure guilty verdicts 是一个完整的陈述句，不缺任何成分，因此需要往空格中填入的连词也应该是不充当任何成分的。显然只有选项 D 满足这一要求。这里的 that 从句是一个同位语从句，其先行词是 concerns ，所以这句话本来的结构应该是 Concerns (that witnesses might be encouraged their stories in court to ensure guilty) were raised. 这里由于 that 从句比谓语 were raised 结构复杂而长，所以将谓语部分 were raised 提前，从而造成同位语从句与其先行词 concerns 被分隔开的现象。此外还要提醒读者注意的是，名词从句中的连词 that 不充当从句成分，但是定语从句中的关系代词 that 是充当从句成分的。
精品译文 人们日益担心，证人可能会因此而受到鼓励，在法庭上夸大其词以保证陪审团对被告做出有罪的判决。
请读者深刻理解名词从句的本质：三种句子（陈述句、一般疑问句和特殊疑问句）充当四种成分（主语、宾语、表语和同位语），于是便构成了四种名词从句（主语从句、宾语从句、表语从句和同位语从句）。同时请掌握各类连词的用法特点，包括是否作句子成分、是否有含义、是否可以省去等。为此，请读者做下面的练习。
Exercise 1.1
1. 将括号中的句子进行适当的句式变换后填入空格中。
1. There is a popular saying______(Family instability causes social instability.)
2. No one knows exactly ______. (Is there life on other planets?)
3. No one knew______. (Would interest rates rise?)
4. I am interested in the question______. (Will people live on the moon someday?)
5. No one knows for sure______. (Will the gains be greater than the losses?)
6. My question is______. (Do we have enough time to go to the movie?)
7. September 11, 2001 started out as an ordinary day, but______ will be forever etched （铭刻） in our memories. (What happened that morning?)
8.______is wholly new kind of police force. (What do we need?)
9. Children often ask______. (How do bears survive winter?)
10. ______is true. (What did he tell you?)
11.______ has not yet been decided. (Who will chair the meeting?)
12. ______has not yet been decided. (When is the meeting to be held?)
13. ______is still a mystery. (Why did he refuse to cooperate with us?)
14. ______is more important than what his fate is. (How does a person master his fate?)
15. Could you tell me______? (Where is the post office?)
16. We don't know______. (Why didn't he come yesterday?)
17. This is______. (Where does our basic interest lie?)
18. My question is______. (Where are we going to have our holidays?)
2. 选择及填空题（ what/which/that 辨析）。
19. He didn't live up to______had been expected of him.
A. what
B. which
C. that
D. all what
20. I realized ______ I said was not exactly ______ I meant to say.
A. what, that, what
B. that, what, what
C. that, which, which
D. that, that, what
21. Although Anne is happy with her success she wonders ______ will happen to her private life.
A. that
B. what
C. it
D. which
22. The mere fact ______ most people believe nuclear war would be madness does not mean that it will not occur.
A. what
B. which
C. that
D. why
23. One of the qualities ______ (that/what/who/why) separate us two-legged animals from the four-legged ones is compassion. It is ______ (which/that/what) makes us stand up tall instead of crawling about on all fours. And standing up tall is ______ (which/that/what) frees our arms to reach out to a fellow being and say, "Let me help you."
24. Change is ______ (which/that/what) keeps us fresh and innovative. Change is ______ (which/that/what) keeps us from getting stale. Change is ______ (which/that/what) keeps us young.