英语语法:名词性从句最详分析
名词性从句是由if, whether, that 和各种疑问词充当连接词所引导的从句，其功同名词一样。
一、主语从句
主语从句是在复合句中充当主语的从句，通常放在主句谓语动词之前或由形式主语it代替，而本身放在句子末尾。
1. It 作形式主语和it引导强调句的比较
It 作形式主语代替主语从句，主要是为了平衡句子结构，主语从句的连接词没有变化。而it引导的强调句则是对句子某一部分进行强调，无论强调的是什么成分，都可用连词that。被强调部分指人是也可用who/whom。例如：
It is a pity that you didn't go to see the film.
It doesn't interest me whether you succeed or not.
It is in the morning that the murder took place.
It is John that broke the window.
2. 用it 作形式主语的结构
(1) It is +名词+从句
It is a fact that …事实是…
It is an honor that…非常荣幸
It is common knowledge that…是常识
(2) it is +形容词+从句
It is natural that…很自然…
It is strange that…奇怪的是…
(3) it is +不及物动词+从句
It seems that…似乎…
It happened that…碰巧…
(4) it +过去分词+从句
It is reported that…据报道…
It has been proved that…已证实…
3. 主语从句不可位于句首的五种情况
(1) if 引导的主语从句不可居于复合句句首。
(2) It is said , (reported) …结构中的主语从句不可提前。例如：
It is said that President Jingo will visit our school next week. (right)
That President Jiang will visit our school next week is said. (wrong)
(3) It happens…, It occurs… 结构中的主语从句不可提前。例如：
It occurred to him that he failed in the examination. (right)
That he failed in the examination occurred to him. (wrong)
(4) It doesn't matter how/whether …结构中的主语从句不可提前。例如：
It doesn't matter whether he is wrong or not. (right)
Whether he is wrong or not doesn't matter. (wrong)
(5) 含主语从句的复合句是疑问句时，主语从句不可提前。例如：
Is it likely that it will rain in the evening? (right)
Is that will rain in the evening likely? (wrong)
4. What 与that 在引导主语从句时的区别
What 引导主语从句时在句时在从句中充当句子成分，如主语.宾语.表语，而that 则不然。例如：
1) What you said yesterday is right.
2) That she is still alive is a consolation.
二、宾语从句
宾语从句就是在复合句中作宾语的名词性从句，通常放在主句谓语动词(及物动词)或介词之后。
1. 作动词的宾语
(1) 由that引导的.宾语从句(that 通常可以省略)，例如：
I heard that be joined the army.
(2) 由what, whether (if) 引导的宾语从句，例如：
1) She did not know what had happened.
2) I wonder whether you can change this note for me.
(3) 动词+间接宾语+宾语从句。例如：
She told me that she would accept my invitation.
2. 作介词的宾语
例如：Our success depends upon how well we can cooperate with one another.
3. 作形容词的宾语
例如：I am afraid (that) I've made a mistake.
That 引导的从句常跟在下列形容词后作宾语：
Anxious, aware, certain, confident, convinced, determined, glad, proud, surprised, worried, sorry, thankful, ashamed, disappointed, annoyed, pleased, hurt, satisfied, content 等。也可以将此类词后的that 从句的看作原因状语从句。
4. It 可以作为形式宾语
It 不仅可以作为形式主语，还可以作为形式宾语而真正的宾语that 从句则放在句尾，特别是在带复合宾语的句子中。 例如：
We heard it that she would get married next month..
5. 后边不能直接跟that 从句的动词
这类动词有Allow, refuse, let, like, cause, force, admire, condemn, celebrate, dislike, love, help, take, forgive等。这类词后可以用不定式或动名词作宾语，但不可以用that引导的宾语从句。例如：
I admire their winning the match. (right)
I admire that they won the match. (wrong)
6. 不可用that从句作直接宾语的动词
有些动词不可用于“动词+间接宾语+that从句“结构中，常见的有Envy, order, accuse, refuse, impress, forgive, blame, denounce, advise, congratulate等。例如：
He impressed the manager as an honest man. (right)
He impressed the manager that he was an honest man. (wrong)
7. 否定的转移
若主句谓语动词为Think, consider, wuppose, believe, expect, fancy, guess, imagine等，其后的宾语从句若含有否定意义，一般要把否定词转移到主句谓语上，从句谓语用肯定式。例如：
I don't think this dress fits you well.(我认为这件衣服不适合你穿。)
三、表语从句
表语从句在复合句中作表语的名词性从句，表语是用来说明主语的身份、性质、品性、特征和状态的，表语常由名词、形容词、副词、介词短语、不定式、动词的-ing、从句来充当，它常位于系动词(be, become, appear, seem,look,sound,feel,get,smell等词)之后。
引导表语从句的词有连词that, whether，连接代词和连接副词，关系代词型what，以及as if, as though, because等连词。
1. 由that引导
The fact is that he doesn’t really try. 事实是他没有做真正的努力。
The trouble is that I have lost his address. 麻烦的是我把他的地址丢了。
My suggestion is that we should tell him. 我的建议是我们应该告诉他。
His sole requirement was (is) that the system work. 他唯一的要求是这个制度能起作用。