1）介詞后面的關系詞不能省略。

　　2）that前不能有介詞。

　　3）某些在從句中充當時間，地點或原因狀語的"介詞+關系詞"結構可以同關系副詞when 和where 互換。例如：

　　This is the house in which I lived two years ago. 這是我兩年前住過的房子。

　　This is the house where I lived two years ago.

　　Do you remember the day on which you joined our club？ 還記得你加入我們俱樂部的那一天嗎？

　　Do you remember the day when you joined our club？