懂英文的朋友請入

這問題要看你在什麼情況下發問.
如果你在電話裡頭問對方是誰, 你可以說 :

Who is speaking?
Who is this?
Who is it?

Who is speaking? 是比較有禮貌的發問方式

如果你想問門外是誰, 你可以說 :

Who is this?
Who is it?
Who is there?

如果你朋友在跟你談論一個人, 而你不能肯定友人在說的是誰, 你可以說 :

Who are you talking about?
Who is that?

如果你在一個場合上看到一個人而你想知道那是誰, 你便可以說 :

Who is that?

p.s. 因為你不肯定是男定女, 所以不太能說 : Who is that guy?
因為 Guy 這詞是用來形容男性的.

除非你在問 這班人是誰? (而這班人中有男亦有女)
這情況下便可以問 :
Who are these guys?

希望解答到你的問題 !

Who's that guy?
(不需要分性別, 男女均可)

Who is this. is more polite.

If you know the gender, you can ask "Who is he or Who is she?".

If you don't the gender of the person, you may ask "Who is it?".

Through the phone, you may also ask "Who is it or Who is speaking?".