1.whom是who的宾格,在正式文体中常作宾语,用作疑问代词或连接代词时非正式文体往往用主格代替,若紧跟在介词之后则只能用whom。若在从句中用作表语则只能用who,不能用whom。
2.whom用作关系代词引出定语从句修饰指人的先行词时意思是“他,她,他们”,在定语从句中作宾语,通常被省略或用who, that代替。
3.whom用于非限制性定语从句中作动词或介词的宾语时不能省略。
4.whom后接由动词to be〔to-v〕构成的复合宾语时,不能用who来代替,且不能省略。

To whom did you give the book?你把书交给谁啦?
The man whom〔that, who〕 I saw is called Smith.我见到的那个人叫史密斯。
These new neighbors, to whom I was introduced yesterday, have come here from Singapore.这些新邻居是从新加坡来的,昨天我被介绍同他们认识了。
I don't know whom to give it to.我不知道给谁。
He is the man (whom) they understood to be extremely rich.他就是那个他们认为腰缠万贯的人。

［词源］＜古英语hwam(谁)