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。
　　　 This is the house in which I lived two years ago.
　　　 This is the house where I lived two years ago.
　　　 Do you remember the day on which you joined our club?
　　　 Do you remember the day when you joined our club?