1、The reasons why he did it are obscure.

他做那事的理由还不清楚。

2、That is why I am not in favour of the plan.

那是我不赞成这个计划的理由。

3、He expounded the reasons why he did it.

他阐述了他做这件事的理由。

4、She knows why the metal is the good conductor.

她知道金属为什么是好的导体。

5、Why he went there is a mystery to me.

他为什么到那里去，这对我来说是个谜。