简单地说就是他们所充当的句子成分是不一样的楼层:2
who 可以做主语,又可以做宾语
whom只能做宾语
who和whom在定语从句中作关系代词时：
1.who指人,在从句中做主语
(1) The boys who are playing football are from Class One.
(2) Yesterday I helped an old man who lost his way.
2.whom指人,在定语从句中充当宾语,常可省略.
(1) Mr.Liu is the person (whom) you talked about on the bus.
(2) Mr.Ling is just the boy whom I want to see.
注意：关系代词whom在口语和非正式语体中常用who代替,可省略