You might want to ask difference between 안^돼 and 아니요. 안^돼 or ~하면 안 돼 is used for saying someone should not do something, and is opposite of 돼 or ~하면 돼 because of '안'. basic form of 돼 is 되다, and '안' makes negative form by preceding '되다' e.g You should not play around parking lot.(주차장 근처에서 놀면 안 돼.) 안되다 means that things aren't going well as someone thought, that person is not as good as people thought, or that quality is lower than standard. For instance you can say "공부가 (잘) 안돼.(Studying isn't going well)" as a opposite of "공부가 잘돼.(Studying is going well.) 시간이 안돼서 ~에 못갔다.(I couldn't go to ~ because I couldn't spare my time.) Tackling usage of 안되다(basic form of 안돼) and 안 되다(basic form of 안 돼) is difficult for quite a few people. So if you see some people use them wrongly, you can correct them. :) 아니요 is for answering interrogative sentence. Have you finished doing your homework?(숙제 다 끝냈니?);No. (아니요.)