定語從句why的用法：
關系副詞why在定語從句中也是用作狀語，表示原因;why引導定語從句時，其先行詞只有一個，那就是the reason;在許多情況下why也可以理解為for which。如：
These are the reasons why we do it. 這些就是我們這樣做的理由。
句中的why we do it為修飾the reason的定語從句，關系副詞why在此相當于for which。
Do you know the reason why she doesn’t like me? 你知道她不喜歡我的原因嗎?
句中的why she doesn’t like me為修飾the reason的定語從句，關系副詞why在此相當于for which。
why是一個比較特殊的關系副詞，它引導定語從句時，除可換成for which外，有時也可換成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.