表语从句
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。
