1)介詞后面的關(guān)系詞不能省略。
    2)that前不能有介詞。
    3) 某些在從句中充當(dāng)時(shí)間，地點(diǎn)或原因狀語的"介詞+關(guān)系詞"結(jié)構(gòu)可以同關(guān)系副詞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?