在复合句中作表语的从句叫表语从句。引导表语从句的关联词与引导主语从句的关联词大致一样，表语从句位于连系动词后，一般结构是“主语+连系动词+表语从句”。可以接表语从句的连系动词有：be, look, remain, seem等。注意：关联词不能省略，并且从句用陈述语序。例如：

The fact is that we have lost the game. That is why he didn't come to the meeting.

需要注意的，当主语是reason时，表语从句要用that引导而不是because。但是reason 后面的定语从句可以用why 或that引导。例如：The reason why he was late was that he missed the train by one minute this morning

【附加】在名词性从句中that与what的区别:

that在名词性从句中不充当句子成分,只起连接作用，没有意义，不能省略（在宾语从句中能省略）。

如：That they are good at English is known to us all.

The problem is that we don't have enough money.

what在名词性从句中充当句子成分，起连接作用，有意义，不能省略。what可以分解成定语从句中的先行词 + 关系代词+ that。理解为双重身份。

如：Do what he says. What (=The thing that) he said was true.