where
1.I don't know where I can buy a football.我不知道哪里可以买到足球。
2.I need to know where I can find him.我需要知道哪里能找到他。
when
1. I don't know when I can get a computer.我不知道我何时能有一台电脑。
2.I want to know when he can get home.我想知道他什么时候能到家。
why
1.I want to know why he was late this morning我想知道他今早为什么迟到了
2.I don't know why he didn't come today.我不知道他为什么今天没来。