在主从复合句中，主语从句、宾语从句、表语从句和同位从句的作用和名词相同，分别作主语、宾语、表语和同位语，因此这四种从句统称名词性从句。引导名词性从句的关联词一般包括that，if 和 whether；连接代词 who，what，which，whatever，whoever，whichever 及连接副词 when，where，how，why，whenever，wherever 等。用法要注意以下几点：
1、主语从句的注意点
1) that 引导主语从句时，如果不是为了强调，一般要以 it 作形式主语，而把主语从句放在主句之后。如：
That air has pressure was known long ago．可写成：
It was known long ago that air has pressure．
很早以前，人们就知道空气有压强。
除此之外，其它的连接代词或副词引导的主语从句，也可以用 it 作形式主语。如：
It hasn't been made clear when the new building is to be built．
新楼什么时候动工还没有明确宣布。
2)如果主语从句是疑问句，只有用 it 作形式主语。如：
Has it been announced when the plane is to take off？
飞机什么时候起飞通知了没有？
2、宾语从句的注意点
1)在宾语从句后有补足语时，一般要用 it 作形式宾语，而将从句放在补足语之后，以求句子结构上的平衡。如：
I think that he should do the test necessary．应为：
I think it necessary that he should do the test．
我认为有必要让他做这个实验。
2)某些表示“心理活动”的形容词，如 sure，happy，glad，
certain，afraid 等后面，也可以接 that 引导的宾语从句。如：
I'm sure that our experiment will succeed．我相信我们的实验会成功。
I'm afraid that she will not come．恐怕她不会来了。
3)在某些动词，如 think，expect，suppose，believe，guess
等后的宾语从句中，如果是否定形式，其否定副词 not 要提前到主句谓语动词上。
I don't think the answer is right．我觉得这个答案不对。
I don't believe that he is a dishonest man．我相信他是一个诚实的人。
但是如果从句中的否定副词不是 not，而是 never，hardly，sel-dom，little 等时，则不遵守这一规则。如：I think he will never succeed． ( 我认为他永远不会取得成功。 )就不能变成：
I never think be will succeed．
如果宾语从句是一个反意疑问句，其反问句部分的主语和谓语形式一般要和主句中的主语和谓语在人称和数上保持一致；
但当是“I don't think ／ believe ／ suppose ／ expect 等＋ that....”结构时，反问句部分的主语和谓语要和从句中的主语和谓语在人称和数上保持一致，且要用肯定形式。如：
He says that Tom is a good student，doesn't he？他说汤姆是个好学生，对吗？
I don't think that he will pass the examination，will he？我想这次考试他不会及格，对吗？
如果宾语从句中有某个含有否定意义的副词或形容词，如hardly，never，seldom，no，few，little 等，其反意疑问句要用肯定形式。如：
We find that he seldom goes to visit his teacher，does he？
我们发现他很少去看望他的老师，对吗？
主句是疑问句时，宾语从句要用陈述句语序。如：
Excuse me，would you tell me when the sports meet is to takeplace？
对不起，你能告诉我这次运动会什么时候举行吗？
但 what'sthematter 作宾语从句时，则不能写成 whatthematteris。因为其中的 what 本身就是主语。如：
Do you know what is the matter with him？你知道她怎么了吗？
3、表语从句的注意点
1)表语从句可用上述各种连词引导。除此之外，还可以用 as引导。如：
It may be as you say．这也许是象你说的那样。
2)也可以用 as if 引导。如：
He looks as if he were going to cry．他看上去似乎就要哭了。
3)还可以用 because 引导。如：
Is it because we are closer to the sun in summer than we are inwinter？
这是由于我们在夏天离太阳比冬天更近吗？
4)宾语从句中的连词 that 有时可以省略，但表语从句中的that 一般不能省略。否则，会造成意义上的混乱。如：
这就是说他太小还不能去上学。
【误】This is he is too young to go to school．
【正】This is that he is too young to go to school．
4、同位语从句的注意点
1)同位语从句和定语从句的判别某些含有一定内容的名词，如：fact，idea，news，hope，thought，promise，suggestion，belief，doubt，truth，order 等词之后，that 所引导的从句一般都为同位语从句。判别的方法主要是看 that 从句在句中是否充当一定的成分。如果 that 只是起一种连接的作用，即所引导的从句只是对前面的名词起一个补充说明作用时，为同位语从句；如果 that 在从句中充当了一定的成分，便是定语从句。如：
The facts that she had collected was enough to prove his guilt．
她所收集的事实材料足以证明他的罪行。
该句为定语从句。因为连词 that 不但起了连接主从句的作用，
而且在从句中作 collected 的宾语。
They were surprised at the fact that China did it all on its own．
所有这些都是中国靠自己的力量干的，这个事实使他们感到大为吃惊。该句是同位语从句。that 在从句中只起了一个连接主从句的作用，在从句中不充当任何成分。
2)同位语从句有时也可以由 that 以外的其它连词引导。如：
Then arose the question where we were to get the machine need-ed．
然后就产生了这样一个问题，我们上哪儿去找所需要的机器。
3)有时同位语从句不是紧跟在先行词后面，而是被其它词隔开，我们称之为分隔形式的同位语从句。如：
The story goes that William Tell did kill the tyrant with that ar-row．
传说威廉·泰尔后来果真用这支箭射死了那个暴君。
5、whether 和 if 在名词性从句中的用法判别
1)在引导表语从句和同位语从句时，只用 whether 不用 if。如：
The question is whether the film is worth seeing．
( 表语从句 ) 问题是这部电影是否值得一看。
The news whether our team has won the match is unknown． ( 同位语从句 )
我们的代表队是否已赢得了比赛，这个消息尚未可知。
2)可以说 if.. .or not，whether.. .or not 或 whether or not，但不能说 if or not。如：
I don't know whether or not he is going to see the film．我不知道他是否打算去看电影。
3)动词“doubt”在表示“怀疑”时，其后的宾语从句有两种情况：
①如果主句谓语动词为肯定式，要用 whether 或迂引导宾语从句。如：
I doubt whether ／ if he will come soon．我怀疑他能否很快就来。
②如果主句为疑问或否定式，则要用that来引导宾语从句。如：
I don't doubt that he will come soon．我毫不怀疑他很快就来。
Do you doubt that he will come soon？你是否怀疑他很快就来？
但当动词“doubt”不表示“怀疑”，而表示“认为……不必；不可能”时，则不遵循这个规则。尽管主句是肯定句，也照样用that 来引导宾语从句。如：
I doubt that there would not be much rain in this area in such sea-son．
我认为在这样的季节该地区不会太涝。
Exercise：
选择填空。
1． they have won the game made us excited．
A．／ B．That C．What D．Where
2． I accept the gift or refuse it is none of your business．
A．If B．Whether C．Even if D．When
3． he says in his report is a very interesting question．
A．What all B．All what C．What D．What that
4．When they will start not been decided．
A．have B．is C．does D．has
5． certain that his invention will lead to the developmentof production．
A．That is B．This is C．It is D．It has
6． is unknown to us all．
A．Where did she put it B．Where she put it
C．That where she put it D．In which she put it
7． nothing to do with us．
A．What she did have B．What she did is
C．What did he do has D．What she has done has
8．The trouble is we are short of hands．
A．what B．that C．how D．which
9．Energy is makes things work．
A．what B．everything C．something D．that
10．Myhometownisnot itusedto tenyearsago．
A．when，do B．what，do C．what，be D．when，be
11．The reason I have to go is my mother is ill in bed．
A．why，why B．why，becauseC．why，that D．that，because
12．The thought he might fail in the exam worried him．
A．which B．that C．when D．so that
13．That is I lost my pen．
A．when B．where C．that D．what
14．Have you any idea ？
A．how fast does light travel B．how quick light travelsC．how soon light traves D．how fast light travels
15．His suggestion to see the art exhibition interestedevery one of us．
A．that we go B．which we should goC．that we would go D．we would go
16．He was interested in he had seen at the exhibition．
A．which B．that C．all what D．all that
17．He often thinks of he can do more for his country．
A．what B．how C．that D．which
18．Hemade quiteclearthathewouldn'tchangehismind．
A．this B．that C．it D．what
19．It has not decided yet they will start．
A．where B．why C．when D．how
20．Hehascome，butIdidn'tknowthathe untilyesterday．
A．is coming B．will comeC．was coming D．wasn't coming
21．He ran back into the room to see if he anything behind．
A．has forgotten B．had forgottenC．has left D．had left
22．They want to know do to help us．
A．what cam they B．what they canC．how they can D．how can they
23．Do you know brighter than the moon？
A．why the sun look B．why the sun looksC．why does the sun look D．why is the sun look
24．Can you tell me the railway station？
A．how I can get to B．where I can get toC．how can I get to D．where can I get to
25．These photographs will show you ．
A．what does our village look likeB．what our village looks like
C．how does our village look likeD．how our village looks like
Key：1． B 2．B 3．C 4．D 5．C 6．B 7．D 8．B9．A 10．C 11．C 12．B 13．B 14．D 15．A
16．D 17．B 18．C 19．C 20．C 21．D 22．B23．B 14．A 25．B
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