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。