作为关系副词，why可引导限制性定语从句，但不能引导非限制性定语从句。若引导非限制性定语从句，则要用for which代替why。例如：

That is the reason why (for which) she refused your invitation. 那就是她拒绝你邀请的原因。

I had told them the reason, for which I didn’t attend the meeting．我把理由告诉了他们，为此我没去开会。