This factory is ______ the one we visited last week. 正确答案是the one,但为什么不是表语从句用what?

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 remained there for an hour.

she has remained where i stood yesterday for an hour.

his suggestion is good.

his suggestion is that we should stay calm.

the question is confusing.

the question is when he can arrive at the hotel.

who will travel with me to beijing tomorrow.

why he cried yesterday.

how i can persuade her to join us in the party.

whether the enemy is marching towards us. 

二 注意：

a 表语从句一定要用陈述语序。

false: the question is when can he arrive at the hotel.

right: the question is when he can arrive at the hotel.

b 不可以用if，而用whether 连接表语从句(as if 例外）。

false: the question is if the enemy is marching towards us.

right: the question is whether the enemy is marching towards us.

right: it looked as if he had understood this question.

c 不像宾语从句，在有表语从句的复合句中，主句时态和从句时态可以不一致。

right: the question is who will travel with me to beijing tomorrow.

right: the question is why he cried yesterday.

d that在表语从句中不可以省掉。

表语从句只能置于主句之后，而主句的动词只能是联系动词。