为什么有的宾语从句引导词只能用that

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。