The meaning of whom

Who和whom都是詢問人物的疑問詞，基本上who是問主語(subject)而whom是問賓語(object)。

Examples:
Q: Who has eaten the apple? A: Tom has eaten the apple. (要求的答案Tom是subject，所以用who問。)
Q: Whom are you talking to? A: I'm talking to Peter. (要求的答案Peter的object，所以用whom問。)

不過在現代英語中，用who比whom更普遍，即"Whom are you talking to?"多變成"Who are you talking to?"。

不過，在子句(clauses)中，whom卻不能被who取代。

Examples:
1. I will only tell the secret to that person. I can trust that person. --> I will only tell the secret to whom I can trust.
2. I have fallen in love with her. I met her in Bangkok last week. --> I have fallen with love with whom I met in Bangkok last week.

以上兩句，that person和her都是object，所以變成clause的時候用whom來指示。這些時候whom不能用who所取代。