1.多数情况下,关系代词whom,which在从句中作介词宾语时,可以跟介词一起放在从句与主句之间（that,who不可以）,也可以把介词放在从句中有关动词的后面,使关系代词紧跟它所修饰的先行词.如：
He is a man of rich experience,from whom much canbe learned.
= He is a man of rich experience,whom much can be learned from.
他是个经验丰富的人,从他那儿可以学到很多.
The school in which he once worked is a key school.
=The school (which that) he once worked in is a key school.
他曾经工作过的学校是一所重点学校.
2.若介词并不与从句中的动词有关系,则要置前.如：
Fortunately we had a map,without which we would have got lost.
很幸运,我们带了一张地图,如没有的话,我们就会迷路了.
3.如果介词与从句中的动词构成固定搭配（短语动词）,则不能置前.如：
The old man who nobody looked after felt very lonely.
没人照顾的那位老人感到很孤独.
