Whom 只能出现在从句里面或是介词（如 to,from,at,of..）后面.
1． Whom 出现在从句里面时,所有用whom的地方都可以用who代替.Whom 这个词只出现在很正式的书面语.口语不用.
2． Whom 在介词（如 to,from,at,of..）后面时,不可替换.
Who told you the story?（主语）
Who is that man over there?（主语补语）
Whom/ Who did you meet at the airport?（宾语）
Whom did you give the book to?（介词宾语）
whom是who的宾格,在现代英语中一般只用于书面体,在口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?两者都可以语中通常用who,但在介词后只能用whom.