Question

为什么有的宾语从句引导词只能用that

Answer 1

that在宾语从句中不充当任何成分没有任何意义，简单来讲就是个装饰作用，所以我们往往在宾语从句中可省掉that
例如：I told you (that) we won the game.
(that) we won the game这个部分是宾语从句，整个从句是完整句，that在其中没有任何作用
I know who you are.
who you are这个也是宾语从句，但是who就不能省略

Answer 2

引导宾语从句的连词：1。从属连词：that，if ,whether，
2. 疑问代词：who, what, which, whose,
3. 疑问副词：when, where, how, why 等。
除了that 无词意外，其它都有词意。当宾语从句不用连词意思也很完整时，就可用that引导，这也是为什么that可以省略的原因。如：She told us (that) she was ill yesterday.
又如：1.I wonder what he is writing to me about?
2. I can't understand why you faild.
3. I'd like to know when she will come here.
以上三个句子就不能用that引导，因为去掉that，意思不够完整。

Answer 3

宾语从句通常有两种，1）动词+宾语从句；2）形容词+宾语从句
一般来说，第二类宾语从句连词基本为that，如：be sure that...

学习第一类宾语从句时，通常老师都会让学生练习直接引语变间接引语。当直接引语是陈述句时，间接引语的连词为that.
例如：
直接引语： Tom said, " I will read it."
间接引语： Tom said that he would read it.