why與what壹for在意思上都有“為什麽”之意，但它們之間有明顯的區別。why表示原因，回答時用because回答。如: “Why are you；what…for不是表示原因,而是表示動作的目的,常用不定式回答。
what for和why的區別
what for :表目的,表用途,壹般翻譯成"用來做什麽"
why:表原因,壹般翻譯成"為什麽",why壹般都要 用because來回答
eg.A:This is a knife.
B:What for?(=What is this for?這是用來幹啥的)
A:It's used for cutting something.
A:Why do you come here so late?
B:Because my watch is broken.