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who和whom都是人称代词，who为主格人称代词(Subject Pronoun), whom为宾格人称代词(Object Pronoun)。下面是具体用法。
who的用法
who主要用在当先行词表人的情况下，一般在从句中充当主语，但有时也可以作宾语，相当于whom。
如：He's the man who bought these books just now.
The man who I served was wearing a hat.
第一句中的who在从句中作bought的主语，而第二句中的who则作served的宾语。
需要注意的是，当介词位于定语从句的句首时，就不能用who，而要用whom。
如：She is the girl about whom the man talked at the meeting an hour ago.
who也可以用来引导非限定性定语从句。
如：The manager, who comes from Japan, told me to help you with your project.
whom的用法
（1）whom主要用在当先行词表人的情况下，在从句中充当动词或介词的宾语。
如：The boy whom you met on the street just now is one of cousins.
He is the man from whom you ought to ask for help.
（2）whom也可以用来引导非限定性定语从句。
如：The soldiers, some of whom come from Alaska, will be sent to Iraq next week.