What's the difference between who and whom?

[Mario]

English Grammar Tests, Elementary Level

ESL/EFL Test #47 "Question Words", question 3

......... is that guy talking to your sister?

(a) Which
(b) Whose
(c) Who
(d) Whom

English Grammar Tests, Elementary Level

ESL/EFL Test #47 "Question Words", answer 3

Who is that guy talking to your sister?

Correct answer: (c) Who
_________________________

is it who or whom, and why?

Mario

[Canadian45]

The answer is ''Who''. Who is the person doing the talking. (S)he is the subject of the sentence. Who is the word used for the subject.

[Jon]

Hello,

Is this sentence correct?

which huy is talking to her?

Thanks a lot

[Yankee]

.
You can say "Which guy is talking to her?" but the meaning of that question isn't the same as "Who is that guy talking to her?"
.