定语从句why的用法：
关系副词why在定语从句中也是用作状语，表示原因;why引导定语从句时，其先行词只有一个，那就是the reason;在许多情况下why也可以理解为for which。如：
These are the reasons why we do it. 这些就是我们这样做的`理由。
句中的why we do it为修饰the reason的定语从句，关系副词why在此相当于for which。
Do you know the reason why she doesn’t like me? 你知道她不喜欢我的原因吗?
句中的why she doesn’t like me为修饰the reason的定语从句，关系副词why在此相当于for which。
why是一个比较特殊的关系副词，它引导定语从句时，除可换成for which外，有时也可换成that，甚至省略。如：
他失去工作的主要原因是他喝酒。
正：The main reason why he lost his job was that he drank.
正：The main reason for which he lost his job was that he drank.
正：The main reason that he lost his job was that he drank.
正：The main reason he lost his job was that he drank.