）介詞後面的關係詞不能省略。
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+關係詞"結構可以同關係副詞when 和where 互換。
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?