表语从句就是放在系统词后面的从句。例如： It seems as if it were going to rain.似乎要下雨。在这句话里面，seem是系动词as if引导的就是表语从句。

The question is where we should put it.问题是我们应该把它放在哪里。

The question is whether his parents will agree to it.

The reason why he was late is that his bike broke down.