Here are some real life examples: 내가 했어. I did it. (It's I who did it) 내가 할게. I'll do it. You will not say 내가 갈게 when leaving, because it means "It's I who will leave". You will say "나 갈게" instead. (Person A and B is talking about their mutual friend, C) Person A : C 좀 달래봐. 너 그런 거 잘 하잖아. Try to comfort C, (you know) you're good at it. Person B : 내가? Am I? 네가, 너가 and 니가 mean the same thing, however 네가 is used only in writing and 니가 is the word that was originally used in southern provinces, and that's now used in 경기도 and 서울 as well(but not so commonly as in southern provinces). 네가 해야지. You should do it. (It's you who should do it) 니가 해라. Do it (yourself). 너가 했어? Did you do it? (Is it you who did it?)