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