关系副词why,在定语从句中作状语,表示原因,why的先行词只能是reason.why可用for which代替.
Is this the reason why (=for which) he refused our offer? 这就是他拒绝我们帮助他的理由吗?
The reason for which he was disissed from the job = the reason why he was dismissed from the job.(作状语,用关系副词)
the reason why he didn't come = the reason for which he didn't come.
because of which = for which