关系代词在定语从句中用作介词宾语时, 介词可放于从句之首, 也可放于从句之末. 但以放于句首较为正式.(介词前置，必须注意不影响动词词组的含义。关系代词who 和 that 用作介词宾语时, 介词必须放在句末.)如:
1. This is the book for which you asked.
=This is the book (that/which) you asked for.
2.Do you know the person with whom I shook hands?= Do you know the person (whom/who/that) I shook hands with?
3. The beginning of agriculture was a big step in human progress with which nothing could compare until our information age.