1、表语从句概述
用作表语的从句叫作表语从句，它位于主句中的连系动词之后。引导表语从句的词有从属连词that、whether、as though(if)；关系代词who, what, which, whom, whose, whatever, whoever, whomever, whichever等；关系副词when, where, why, how, however, whenever, wherever等。可以接表语从句的连系动词由be, look, remain, seem等。That引导表语从句时，在口语中，间或可以省略。
The trouble is that we are short of money.困难是我们资金短缺。
That is why stone walls are used instead of fences around New England fields.这就是为什么在新英格兰用石头墙而不用栅栏的原因。
At that time, it seemed as if I couldn't think of the right word anyhow.当时,我似乎怎么也想不出一个恰当的字眼来。
2、由从属连词that，whether引导的表语从句。
that在引导表语从句时无词义，而whether有词义，意为、“是否”。这时主句的主语常常是些抽象名词，如question(问题)，trouble(麻烦)，problem(问题)，result(结果)，chance(可能性)，suggestion(建议)，idea(想法)，reason(理由)等。表语从句对主句主语进行说明、解释，使主语的内容具体化。
The trouble is (that) she has lost his money. 麻烦的事是他丢了钱。
The question is whether we need more ice cream.问题是我们是否还需要一些冰淇淋。
The problem was that it was too valuable for everyday use.问题是它作为日常之用太贵重了。
What she couldn’t understand was that fewer and fewer students showed interest in her lessons. 我们不能理解越来越少的学生对他的课不感兴趣。
3、由关系代词引导的表语从句。
关系代词who, what, which, whom, whose, whatever, whoever, whomever, whichever等引导表语从句，在句中作主语、宾语、表语，关系代词不能省略。
The question is which of us should go.问题是我们哪一个应该去。
The problem was who could do the work.问题是谁能做这项工作。
That's what he is worrying about.那就是他在担心的事。
That's what we should do.那是我们应该做的。
4、由关系副词引导的表语从句。
Go and get your coat. It's where you left it. 去把雨衣拿来。就在你原来放的地方。
I had neither a raincoat nor an umbrella. That’s why I got wet through. 我们既没伞也没雨衣，这是我们淋湿的原因。
That is how mice ruin many stores of grain every year.那就是老鼠是怎样每年损害大量粮食的。
That is what he is worried about.那就是他所担心的。
5、由连词because，as if/as though等引导的表语从句。
It looked as if it was going to snow.看起来好像要下雪了。
That's because we never thought of it.这是因为我们从未想过此事。
It seems as if he didn’t know the answer.好像他不知道答案。