who 可以做主語,又可以做賓語
whom 只能做賓語
例子：1,who are you?不能是whom are you?
2,This book belongs to whom/who?兩者都可以
1.who指人,在從句中做主語
(1) The boys who are playing football are from Class One.
(2) Yesterday I helped an old man who lost his way.
2.whom指人,在定語從句中充當(dāng)賓語,常可省略.
(1) Mr.Liu is the person (whom) you talked about on the bus.
(2) Mr.Ling is just the boy whom I want to see.
注意：關(guān)系代詞whom在口語和非正式語體中常用who代替,可省略
簡單點(diǎn)說就是：
Whom 只能出現(xiàn)在從句里面或是介詞（如 to,from,at,of..）后面.
1． Whom 出現(xiàn)在從句里面時(shí),所有用whom的地方都可以用who代替.Whom 這個(gè)詞只出現(xiàn)在很正式的書面語.口語不用.
2． Whom 在介詞（如 to,from,at,of..）后面時(shí),不可替換.