do you know the reason ___they didn't agree with us

Do you know the reason ___they didn't agree with us?
这就是用why
固定一下：
the reason why +结构完整的定语从句.
why相当于for which
也就是for the reason
（另外：也有些语法书说用that的,that they didn't agree with us是同位语从句）
还有：
当引导词做定语从句的宾语时,用that/which/不填
如：
I don't know the reason that/which/不填 he gave us.

您好！reason 是名词，这里是先行词。 why 是关系副词，引导原因状语从句。 望采纳 ！谢谢！