从这两个单词本身的意思可以判断出来。what什么; which哪一个 它们都可以在名词性从句中做主语或宾语。但what 不表示同一范畴内的东西,which表示同一范畴内的“哪一个”。 There are so many things in the shop,and I don’t know what I can buy(=what to buy). There are so many interesting books in the bookstore,and I don't know which I can buy(=which to buy).