在什么情况下,who不能取代whom

一般来说,如果在句中（包括主句和从句）,其充当介词的宾语并紧紧放在介词之后时,则只能用whom,而不能用who.
如：Who are you talking to?= Whom are you talking to?
但是只能说：To whom are you talking?
再如：Do you know the student (who/whom) I talked to just now?
= Do you know the student to whom I talked just now?
而不能说：Do you know the student to who I talked just now?

who为主语:
Is he the man who wants to see you?
whom为宾语:
He is the man whom that I saw yesterday.
一般可以用who代替whom

whom是who的宾格，但是现在一般情况下都可以用who代替whom