表语从句的定义

This is the house where I lived before.肯定不是表语从句。就如你的理解，是定语从句在修饰表语house.很多参考书都有写纰漏或错误之处嘛。表语从句就是用一个句子作为表语。如：
This is what I said yesterday.(what I said yesterday作表语。）
What you need is just what I need.(What you need是主语从句， what I need是表语从句。）
The fact is that she loves me.
The problem is whethar it will rain tomorrow.
小幽ice提供的例句也是。表语从句由that(可省），if/whether, 或wh-引导。

表语从句的位置应该在系动词的后面,你上面的句子中house是名词作表语,它后面的从句是一个定语从句来修饰名词house的.以上的几个才是表语从句
The reason why he didn't go to school is that he was ill.(that引导的是表语从句)
What we want to know is whether we should help him.(whether引导的是表语从句)

表语从句
一 定义：
A 表语从句就是用一个句子作为表语。
The problem is puzzling.
主语 连系动词 形容词作表语
The problem is when we can get a pay rise.
主语 连系动词 一个句子作表语---表语从句
B 连接表语从句的连接词有：that, what, who, when, where, which, why,
whether,how.
He has become a teacher.
He has become what he wanted to be ten years ago.
She has