Question

名词性从句和定语从句的问题

I don't believe the reason that he explained to me.我知道这句that引导定语从句
可是如果把that 换成what 感觉也很通啊，what引导同位语从句 求解释

Answer 1

what 不能引导定语从句，只能引导名词性从句，而这里主句不缺成分，故不用what。
这里的that 做定语从句的连词，先行词为reason,在从句中作宾语，所以这里that 也可以省略不填。
要注意如果对reason做补充说明，that做同位语的引导词，不做成分。
如：I don't beliebe the reason (that) he told me. 定语从句
i don't beliebe the reason that he was too busy. (同位语）
i don't believe what he told me 。（宾语从句）

Answer 2

用what不行，那变成名词性从句了。名词性从句是没有名词置从句前的。what必须在从句中充当句子成份，that在名词性从句中不充当从句句子成份，that在定语从句中才充当句子成份。。
That he said he had been married was not true。
What he said was not true。

Answer 3

这里what引导同位语从句？谁说的 引导定语从句 要么用关系代词that which等 或者关系副词where how when
引导同位语从句是that
你如果用what 它是承前启后的 需要在主句和从句中都充当成分
词句主干部分I don't believe the reason已经是一个完整的句子了
如果要用what 那就应该是I don't believe what he explained to me。
what = the reason that