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?