whom是who的宾格。做主语的时候用who，做宾语的时候用whom，一般前面有介词或者动词。who是用于充当从句的主语,是表示动作的发起人

whom 是用语充当从句的宾语,是表示动作的承受人

比如：The boys who are playing football are from Class One. 这里who做的是从句的主语，所以是用who。

例如：

He is the man who will get married with Mary next Sunday (who主语）

He is the man whom Mary will get married with next Sunday(who宾语）