表语从句讲解：
1， 用that引导的表语从句
The fact is that he is old enough to go to school 事实就是他已经到了去上课的年龄
2， whether引导的表语从句（不能替换成if）
The question is whether he will come to your party 问题就是他是否会来你的聚会。
3， 疑问代词和疑问副词引导的表语从句
This is who you want to know 这个就是你想要认识的那个人。
4， as if ，as though 引导的表语从句（通常跟感官动词等系动词搭配）
It sounds as if a boy is smiling 听起来好像一个男孩在微笑。
It sounds as though a girl is laughing 听起来好像一个女孩在大笑
5， because 引导的表语从句
This is because he can go to school today 这就是因为他今天不能去上课。
