1）介词后面的关系词不能省略。

　　2）that前不能有介词。

　　3）某些在从句中充当时间，地点或原因状语的"介词+关系词"结构可以同关系副词when 和where 互换。例如：

　　This is the house in which I lived two years ago. 这是我两年前住过的房子。

　　This is the house where I lived two years ago.

　　Do you remember the day on which you joined our club？ 还记得你加入我们俱乐部的那一天吗？

　　Do you remember the day when you joined our club？