1． 定义：用作主语的从句叫做主语从句。
2． 构成：关联词+简单句
3． 引导主语从句的关联词有三类：
(1) 从属连词that。 如：That they were in truth sisters was clear from the facial resemblance between them.
很明显，她们确是亲姐妹，她们的脸型很相似。
(2) 从属连词whether。如：
Whether he’ll come here isn’t clear. 他是否会来这里还不清楚。
(3) 连接代词who, whom, whose, what, which, whoever, whatever, whichever
连接副词 where, when, how, why。如：
What she did is not yet known. 她干了什么尚不清楚。
How this happened is not clear to anyone. 这事怎样发生的，谁也不清楚。
Whoever comes is welcome. 不论谁来都欢迎。
Wherever you are is my home ---- my only home. 你所在的任何地方就是我的家----我唯一的家。
1． 定义：用作同位语的从句叫做同位语从句。
2． 用法：同位语从句的先行词多为fact, news, idea, thought, question, reply, report, remark等，关联词多用从属连词that。如：
They were all very much worried over the fact that you were sick.
对你生病这件事，他们都很焦虑。
Where did you get the idea that I could not come?
你在哪儿听说我不能来？
Early in the day came the news that Germany had declared war on Russia.
德国已对俄国宣战的消息一大早就传来了。
注：同位语从句偶尔由从属连词whether引导。如：
I have no idea whether he’ll come or not. 我不知道他是否来。
连接代词who, which, what和连接副词where, when, why, how亦可引导同位语从句。
The question who should do the work requires consideration.
谁该干这项工作，这个问题需要考虑。
We haven’t yet settled the question where we are going to spend our summer vacation.
到哪儿去度暑假，这个问题我们还没有决定。
It is a question how he did it.
那是一个他如何做了此事的问题。
1． 定义：用作表语的从句叫做表语从句。
2． 构成：关联词+简单句
3． 引导表语从句的关联词的种类：
(1) 从属连词that。如：
The trouble is that I have lost his address. 麻烦是我把他的地址丢了。
(2) 从属连词whether, as, as if。如：
He looked just as he had looked ten years before. 他看起来还与十年前一样。
The question is whether they will be able to help us. 问题是他们是否能帮我们。
注：从属连词if一般不用来引导表语从句，但as if却可引导表语从句，如：
All this was over twenty years ago, but it’s as if it was only yesterday.
这都是20多年前的事了，但宛如昨天一样。
能跟表语从句的谓语动词一般为系动词be, seem, look等。如：
It looked as if it was going to rain. 看起来天要下雨了。
(3)连接代词who, whom, whose, what, which, whoever, whatever, whichever
连接副词 where, when, how, why。
如：The problem is who we can get to replace her. 问题是我们能找到谁去替换她呢。
The question is how he did it. 问题是他是如何做此事的。
That was what she did this morning on reaching the attic. 那就是她今晨上了阁楼干的。
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