1. verb stem + 고 있다 : present progressive form e.g. 말하다 (to talk) : 말하(verb stem) + 고 있다 --> 말하고 있다 (to be talking) 2. verb stem + 냐+ ? : It is a casual form of a question usually used in between friends or to younger people to ask about a fact or a situation. '냐' is only used in speech. 너 나에게 말하고 있냐? ( Are you talking to me?) : This may sound offensive depending the speaker's tone of speech. So I'd like to recommend you to use a different form in casual setting. And '한테' is more commonly used in speech instead of '에게'. '에게' is more used in writing. e.g. 너 나한테 말하고 있어? 너 나한테 말하고 있니? 3. 당신은 저에게 말하고 있습니까? : This is a deferential style so it sounds very formal. In Korean, 당신 is not commonly used in speech. 당신 is used in between married couples or in poems or song lyrics. If you call a stranger 당신, it sounds offensive. People call the listener's name or their social position instead of directly mentioning 'YOU'. Your sample sentence may be used in advertisement to mention you (audience) but you may want to use another form to ask someone politely if they are talking to you. e.g. 저한테 말하고 있어요? (polite) 저한테 말씀하고 계세요? (honorific) cf. 저한테 말씀하시는 거예요? : You can also use this form to ask for clarification.

if you would like to see the difference, you should care about the situation when they are used as "있다" is used in various ways :) first one, 너 나에게 말하고 있냐? mean are you talking to me? so ~하고 있다 means " doing " , anything which has ~ing form e.g. are you eating lunch now? 너 지금 점심 먹고 있냐? second one is also the same meaning but in the FORMAL way 당신은 저에게 말하고 있습니까? well, speaking of you wanting to speak naturally, i wouldn't recommend you to say the latter one. there must be some situations you could speak like that but in common conversations, i don't think you will often encounter it. NDI :)