主语从句
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知识点总结
1.定义
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。
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。被强调部分指人是也可用who/whom。例如：
It is in the morning that the murder took place.
It is John that broke the window.
3.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…
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…
4.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)
5.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.
精选问题
【例题1】Which (apple) he chooses is night。这句话为什么从句缺宾语在which后加个名词就行呢？
解：Which (apple) he chooses 是主语从句，如果前面有选择的范围，直接用which是可以的，如果用what...
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【例题2】It was a pity that the weather was so bad. 请问用a pity 修饰的主语从句不是应该虚拟吗？为什么从句还是用was不是用的（should）+原形？
解：假如与客观事实相符的时候，不一定用虚拟语气...
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【例题3】_______we'll go camping tomorrow depends on the weather.
A.If B.whether C.that D.where
解：本题是考察whether引导主语从句的用法!
if和whether都有“如果、要是、是否”的意思，他们都可以引导条件状语从句...
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