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2017高考英语正在紧张地备考中，英语有哪些知识点是需要你掌握的呢?接下来小编为你整理了，一起来看看吧。
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.
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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