who和whom都是人稱代詞，who為主格人稱代詞(Subject Pronoun), whom為賓格人稱代詞(Object Pronoun)。下面是具體用法。

who主要用在當先行詞表人的情況下，一般在從句中充當主語，但有時也可以作賓語，相當于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在從句中作bought的主語，而第二句中的who則作served的賓語。

需要注意的是，當介詞位于定語從句的句首時，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也可以用來引導非限定性定語從句。

如：The manager, who comes from Japan, told me to help you with your project.

（1）whom主要用在當先行詞表人的情況下，在從句中充當動詞或介詞的賓語。

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也可以用來引導非限定性定語從句。

如：The soldiers, some of whom come from Alaska, will be sent to Iraq next week.

來源：高三網