英语的语法中，复合句有三大类型：定语从句，状语从句和名词性从句由于名词性从句中的宾语从句 在之前讨论过，本文讨论剩下的主语从句，表语从句和同位语从句。
主语从句
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从句在句子中做主语。
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引导词通常是连词that,whether,who,what,whatever等
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通常按第三人称单数对待
That
句子成分完整的时候，用that引导。
从句和主句的变化可以组合出不同的句子。
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That you don't know the rule is not an excuse for your failure. 你不知道规则并不能成为你失败的理由。
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That real estate price will go down is certain. 房地产价格下降将是必然。
Whether
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Whether she will get married or not is still unknown
who whoever whom whose what whatever which whichever
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Who will win the match is still unknown.
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What he wants to tell us is not clear.
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Whatever you do is not my primary concern. 你做任何事都不是我们首要关心的
when where how why
When will he get married remains unknown.
Why you are so clever is a puzzle we all want to work out
It作形式主语It be 形容词 that从句
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It is certain that real estate price will go down
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It is true that she will get married in winter.
It be 名词词组 that从句
no wonder, a pity, a honour, no surprise
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It is a pity that you will married him, not me. 很可惜，你要嫁的是他不是我。
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It is an honor that you come to our little party.
It be 过去分词 that从句
said，believed，announced, expected
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It is said that she has been pregnant for 3 months. 据说她已经怀孕三个月。
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It is believed that success comes from hard work.
It doesn't matter (makes no different) 连接代词或副词引导的主语从句
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It doesn't matter when he will arrive.
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It makes no difference whether he will come to the part or not.
表语从句
如果不懂表语是什么，请先复习
从句在主系表结构的主句中充当表语成分
常用连接词：that, what, who, when, where, which, why, whether, how, whoever，whichever ，whatever
reason 用that引导
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The reason why he was late was that he missed the train by one minute this morning.
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The fact is that we have lost the game.
可用be，look，seem, sound, appear引导
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It looks as if it is going to rain.
whether可引导表语从句，但与之同义的if却通常不用于引导表语从句。
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We know things will turn out to be better. The question is whether we can survive to see that day. 我们都知道形势终将变好，问题是我们能不能或者看到那天。
That is because …指原因或理由。
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He did not drink with us last night. That is because he had to help his little sister with her homework.
That is why … 指由于各种原因所造成的后果。
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He was married. That is why he refused your proposal.
同位语从句
从句充当同位语成分，起解释说明作用
被解释说明的词和同位语在逻辑上是主语和表语关系
主语you和同位语the elder brother就是主谓关系：you are the elder brother. 同位语elder brother是对you的解释。
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You, the elder brother, should take care of your sister.
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The question who can do the work requires consideration.
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You must answer the question whether he agrees with the plan or not.
区分定语从句同位语从句
主语和同位语是主表关系，主从句间加is说得通就是同位语，不通是定语.
The news that he told me yesterday is true.
上句是定语从句， he told me是用来限定the news的。
That fact that you are late surpised everyone. You have never been late before. 你迟到的事实惊呆了所有人。以前从来不迟到。
The fact is you are late,说的通，所以是同位语。
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