【定义】一个句子在另一个句子中充当主语、宾语、表语、同位语，相当于名词，故称为名词性从句。
四大从句】名词性从句分为主语从句、宾语从句、表语从句和同位语从句。
一、主语从句。
一个句子在另一个句子中充当主语，即句子充当主语，该句子称为主语从句。
完成例句】
根据句意选词填空(that, if, whether, why, when, what)
(1) that he will succeed is certain.
(2) whether he will go there is not known.
(3) what he said is not true.
(4) when the meeting will be held has not been decided yet.
结论】 ①that－从句，从句为一个句意完整的陈述句，没有具体意义，不充当句子成分，只起标志性作用，但that 不能省略。
②whether －从句，从句**于一般疑问句，whether不能省略，意为“是否”，不能用if替换。
③特殊疑问词－从句，从句**于特殊疑问句，特殊疑问词不能省略，意思为特殊疑问词本来的意思，在从句中充当句子成分。
疑难1】who he is is none of my business.
疑难剖析1】任何从句要用陈述句语序，即主谓语序。
疑难2】that he likes english is known to us all.
what he said at the meeting made me sad.
疑难剖析2】主句时态为过去时，从句时态为过去的某一时态；主句时态为现在时或将来时，从句时态视具体情况而定。
疑难3】that 2010 asian games are successful is known to all.
疑难剖析3】单个句子作主语，视为单数。
疑难3】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.
疑难剖析3】1．主语从句通常由it充当形式主语，而本身放在句子末尾。
2．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。
3. 用it 作形式主语的常用结构。
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。
it is ＋ 形容词 ＋ 从句。
it is natural that…很自然……
it is strange that…奇怪的是……
it is ＋ 不及物动词 ＋ 从句。
it seems that…似乎……
it happened that…碰巧……
it appears that…似乎……
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……
疑难5】it is said that chairman hu will visit our school next week. (
that chairman hu will visit our school next week is said. (
疑难剖析5】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位于复合句句首。
2. it is said / reported…结构中的主语从句不可提前。
3. it happens / occurs…结构中的主语从句不可提前。
it occurred to him that he failed in the examination. (
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。
it doesn't matter whether he is wrong or not. (
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。
is it likely that it will rain in the evening？(√
is that will rain in the evening likely？(×
疑难6】 what 与that 在引导主语从句时的区别。
what you said yesterday is right.
that she is still alive is a consolation.
疑难剖析6】 what 引导主语从句时在从句中充当主语、宾语或表语，而that则不充当任何成分。
二、宾语从句。
在复合句中作宾语的名词性从句是宾语从句，通常放在主句谓语动词 (及物动词)或介词之后。
完成例句】根据句意选词填空(that, if, whether, where, what，who)
(5)i heard that he joined the army.
(6)①she did not know what had happened.
②i wonder whether you can change this note for me.
(7)she told me that she would accept my invitation.
结论9】①由that引导的宾语从句(that 通常可以省略)，that在从句中不充当任何成分。
②由what引导的宾语从句，what在从句中充当主语、宾语或表语。
③whether (if)引导的宾语从句，在从句中不充当任何成分，只起连接作用，意为“是否”。
疑难1】引导宾语从句时if和whether的区别。
i asked her if / whether she had a bike.
they discussed whether they will go back right now.
we're worried about whether he is safe.
i don't know whether or not he will come.
i don't know whether to go.
疑难剖析1】引导宾语从句时用if和whether都可以，但有只用whether的特殊情况：动词discuss后；介词后；whether与or not连用时； whether to do。
疑难2】i am afraid (that) i've made a mistake.
疑难剖析2】作形容词的宾语时，可以省略that。
注意】下列形容词后常跟that引导的从句作宾语：
anxious, aware, certain, confident, convinced, determined, glad, proud, surprised, worried, sorry, thankful, ashamed, disappointed, annoyed, pleased, hurt, satisfied, content等。(有时也将此类词后的that 从句看作原因状语从句。 )
疑难3】we heard it that she would get married next month.
疑难剖析3】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)。
疑难4】i admire their winning the match. (
i admire that they won the match. (
疑难剖析4】后边不能直接跟that 从句的动词有：
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。
疑难5】he impressed the manager as an honest man. (
he impressed the manager that he was an honest man. (
疑难剖析5】有些动词不可用于“动词＋间接宾语＋that－从句”结构中，这类动词常见的有envy，order，accuse，refuse，impress，forgive，blame，denounce，advise，congratulate等。
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