(1)介词放在句末时,who,that,which可省
但介词在关系代词前时,只能用“介词+which/whom”结构.例如：
This is the house to which we lived last year.
=This is the house which we lived last year to
Please tell me to whom you borrowed the English novel
=Please tell me whom you borrowed the English novel to
(2).含有介词的固定动词词组中,介词不可前置,只能放在原来的位置上.
This is the person whom you are looking for.这就是你要找的那个人.