The truth is (that I have been there)为什么是表语从句

一则be动词后面是表语从句，一则宾语从句是修饰宾语的，也就是说必须有宾语才行。

如I

like

reading

books

which

are

written

by

BingXin.

which

从句修饰宾语books，就是宾语从句。

或者在举一个be动词的例子，你会更明白。

He

is

a

good

student

who

usually

finishes

homework

on

time.

不知道你明白没有，宾语从句一定要有宾语才行。