一、在英文中，名词或名词短语主要充当四种成分，主语、宾语、表语和同位语。
如果我们把句子当作名词来使用，分别在另一个句子中作主语、宾语、表语和同位语(见下表第二列)，于是就构成了四种从句：主语从句、宾语从句、表语从句和同位语从句。因为以为这四种从句在本质上相当于名词的作用，所以我们把他们统称为名词从句。请看例句：
二、三种句子来充当四种成分。(3，4)
上面说到名词从句就是用一个完整句子充当另一个句子的某种句子成分便构成了具体某一名词从句。那么可以用什么样的句子来充当四种成分呢？答案是：
三、陈述句可分别充当四种句子成分(名词从句的重点内容)，要在陈述句首加上他that。
对于陈述句，我们要在句首加上that,然后把“that+陈述句”分别放在另一个句子的四种位置即构成四种名词从句。
1.主语从句。“that+陈述句”在主语位置即成主语从句：
Stuart is ugly.
That Stuart is ugly is a fact..
That Malaysians don’t speak good English is obvious.
更常见的是用it式主语置于句首，而将主语从句放在句末。例如：
It is a fact that Stuart is ugly.
It is obvious that Malaysians don’t speak good English。
因此下面都是常见的主语从句句型：(￥)
①It is +过去分词 +that 从句(found, believed, reported, thought, noted…)
②It is +形容词 +that 从句(clear, possible, likely, natural, certain, fortunate, necessary, strange…)
③It is +名词(短语) +that 从句(a pity, a shame, an honor, a good idea, a miracle…)
2. 宾语从句。“that+陈述句”在宾语位置即成宾语从句：
I think (that) you turned off the light.
We know (that) women love shopping.
She believed (that) her child was premature.
只有宾语从句中的that才可以省去，其他名从that不省。
3.表语从句。“that+陈述句”在表语位置即成表语从句：
My idea is that the students should be more open to each other.
The reason is that he was careless and irresponsible.(考点)
4.同位语从句。“that+陈述句”在同位语位置即成同位语从句：
所谓同位语，就是用来补充说明名词的成分。
The rumor that Tom washed his socks once a month turned out to be untrue.
Our fathers brought forth upon this continent a new nation, conceived in liberty and dedicated to the proposition that all men are created equal.
A saying goes that tomorrow is another day.
四、一般疑问句可分别充当四种句子成分。1、一般疑问句语序变成陈述语序。2、用来whether或if引导。
1.主语从句: does she love dog-walking?
Whether she loves dog-walking or not is unknown.
2.宾语从句:
I don’t know if/whether whether she loves dog-walking
3.表语从句:
My concern is whether you are a student or not.
4.同位语从句:
They are faced with the problems whether they should continue to play.
补充：if whether区别
1)if一般用于引导宾语从句，而whether可引导所有名词从句
If he comes or not makes no difference.
The question is if he will arrive.
2)if 不和or not直接连用，一般不说 if or not。但可以说 if … or not 。而whether没有此限制。
I don’t know if or not he arrives.
I don’t know whether or not he arrives.
五、特殊疑问句可分别充当四种句子成分。特殊疑问句变成陈述句语序。
There arose the question where we could get the loan.
I have no idea what gender he is.
Why he refused to cooperate with us is still a mystery.
I don’t know how many letters I have written to her.
NBA is what I am crazy about.
How China should develop next is a serious problem.
Excuse me, can you tell me where the post office is?
My question is where we are going to have our holidays.
The point is whose book is the most horrible.
Whoever comes will be welcomed.
Whatever is left over is yours
Do whatever you please.
Read whichever books you please.
I’ll give it to whichever of you wants it.
P.S. 最后说明：同位语从句多用that引导，少用whether, if以及代词或副词引导，因为我们多是用陈述句补充名词的内容，而少用一般疑问句或特殊疑问句来补充名词的内容。
温故知新：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。
三种句子：
四种成分：
这三种句子不是直接放在另个句子中作成分，而是要做以下调整：
1)用陈述句作成分时，要在句首加
2)用一般问句作成分时，先将疑问句换成语序，然后在句首加
3)用特殊问句作成分时，只需将疑问句换成 语序。
下面是95年1月份的阅读真题，来感受下几个难句吧，若搞定名词从句，那它们也就是小儿科。
It is a curious paradox that we think of the physical sciences as “hard”， the social sciences as “soft，” and the biological sciences as somewhere in between. This is interpreted to mean that our knowledge of physical system is more certain than our knowledge of biological systems， and these in turn are more certain than our knowledge of social systems. In terms of our capacity of sample the relevant universes， however， and the probability that our images of these universes are at least approximately correct， one suspects that a reverse order is more reasonable
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