在句子中起名词作用的句子叫名词性从句。 名词性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． 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。被强调部分指人时也可用who/whom。
例如：
a) It is a pity that you didn’t go to see the film. 你不去看那场电影真可惜。
b) It doesn’t interest me whether you succeed or not.
我对你成功与否不感兴趣。
c) It is in the morning that the murder took place. 谋杀案是在早上发生的。（强调句型）
d) It is John that broke the window.
是John打碎的窗户。（强调句型）
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…
It appears that… 似乎…
(4) It ＋ 过去分词 ＋ 从句
It is reported that… 据报道…
It has been proved that… 已证实…
It is said that… 据说…
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：
正确表达：It is said that President Jiang will visit our school next week.
错误表达：That President Jiang will visit our school next week is said.
（3）It happens/occurs…结构中的主语从句不可提前。
例如：
正确表达：It occurred to him that he failed in the examination.
错误表达：That he failed in the examination occurred to him.
（4）It doesn’t matter how/whether …结构中的主语从句不可提前。
例如：
正确表达：It doesn’t matter whether he is wrong or not.
错误表达：Whether he is wrong or not doesn’t matter.
（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：
正确表达：Is it likely that it will rain in the evening?
错误表达：Is that will rain in the evening likely?
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：
a) What you said yesterday is right.
b) 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.
我听说他参军了。
(2) 由what, whether (if) 引导的宾语从句，例如：
a) She did not know what had happened. 她不知道发生了什么。
b) I wonder whether you can change this note for me.
我想知道你是否能帮我改一下笔记。
(3) 动词＋间接宾语＋宾语从句。例如：
She told me that she would accept my invitation.
她对我说她会接受我的邀请。
2. 作介词的宾语
Our success depends upon how well we can cooperate with one another.
我们的成功取决于我们之间的合作。
3. 作形容词的宾语
I am afraid (that) I’ve made a mistake.
我恐怕我已经犯了一个错误。
注意：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.
我听说她下个朋就会结婚了。
5. 后边不能直接跟that 从句的动词
正确表达：I admire their winning the match.
错误表达：I admire that they won the match.
6. 不可用that从句作直接宾语的动
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：
错误表达：He impressed the manager that he was an honest man.
7. 否定的转
若主句谓语动词为think, consider, suppose, 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
1) The question is whether we can make good preparation in such a short time
2) This is why we can’t get the support of the people.
3) But the fact remains that we are behind the other classes.
4) The reason he is late for school is that he missed the early bus
四. 同位语从句
同位语从句就是在复合句中作名词的同位语的名词性从句。
1. 同位语从句的功能
同位语从句对于名词进一步解释，说明名词的具体内容，一般由that引导，
例如：
1) The king’s decision that the prisoner would be set free surprised all the people.
2) The order that all the soldiers should stay still is given by the general.
2. 同位语在句子中的位置
同位语从句有时可以不紧跟在它所说明的名词后面，而是被别的词隔开。
例如：
He got the news from Mary that the sports meeting was put off.
3. 同位语从句与定语从句的区别
(1) 定语从句中的that既代替先行词，同时以在从句中作某个成分（主语或宾语），而同位语从句中的that是连词，只起连接主句与从句的作用，不充当句中任何成分。
(2) 定语从句是形容词性的，其功能是修饰先行词，对先行词加以限定，描述定的性质或特征；同位语从句是名词性的，其功能是对名词进行补充说明。
例如：
1) The news that he told me is that Tom would go abroad next year.（他告诉我的消息是汤姆明年将出国。）（第一个that引导的从句是定语从句，that在从句中作宾语）
2)The news that Tom would go abroad is told by him.（汤姆将出国的消息是他讲的。）（同位语从句，that在句中不作任何成分）
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