As you've noticed, '-니' and'-어' can be both used in making questions in casual setting. However you cannot use '-니' for answering while you can both use '-어' for a question and an answer. Here is one more difference between '-니'and'-어'. You can use '-어' to a older person when he/she is very close to you. e.g. 형, 밥 먹었어? 언니, 이거 먹어. (O) However, you CANNOT use '-니' to an older person even if he/she is close to you. e.g. 형, 밥 먹었니? (X)

No, the show I was watching had captions all through out. I heard and it was captioned, the person responding as "네 할수 있니", but thanks for the insight!

While ~니 and ~어 are both informal speech, please note: 1. ~니 is merely a sentence ending; you cannot use it in statements -> 할 수 있어? 네 할 수 있니 is not correct. Perhaps you have mistaken it for ~지, which adds a sense of confidence to the sentence. (할 수 있지 - of course I can / I'm sure I can) 2. ~어 can be used in both questions and statements. So questions can end in both ~니 and ~어. There is a slightly different nuance to both. ~니 sounds more considerate and soft. - 시험 잘 봤니? vs. 시험 잘 봤어? (in the first case, it sounds slightly more careful) ~니 also goes well with 혹시... (perhaps) 혹시 돈 있니? Do you perhaps have money with you? Hope this was helpful!