WHO 可以做主语,又可以做宾语
WHOM只能做宾语
例子：1,who are you?不能是whom are you?
2,This book belongs to whom/who?两者都可以
一.定语从句及相关术语
1.定语从句：修饰一个名词或代词的从句称为定语从句,一般紧跟在它所修饰的先行词后面.
2.关系词：引导定语从句的关联词成为关系词
关系词有关系代词和关系副词.关系代词有that,which,who,whom,whose,as等；关系副词有where,when,
why等.
关系词常有3个作用：1,引导定语从句.2,代替先行词.3,在定语从句中担当一个成分.
二.关系代词引导的定语从句
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