定语从句中以who和whom引导的有什么区别？

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

定语从句中的who和whom叫做关系代词。其中who在定语从句中充当主语，whom充当宾语。但注意：who可以代替whom作宾语，但whom 不可代替who做主语。
The man who is standing there is my teacher.
The man who/whom you saw just now is my teacher.

WHO 代的是从句中的主语。
WHOM代从句中的并语。

比如：
She is the girl whom I am falling in love with.
Shi is Mary who is my mother.