Two forms of negation, both placed immediately before a verb or adjective:
안 – you didn’t do it, but you had the ability to
못 – you couldn’t do it because you don’t have the skills or ability
우유를 못 마셔요.
I can’t drink milk. (I’m lactose intolerant)
우유를 안 마셔요.
I don’t drink milk. (I think it tastes awful)
수줍어니까 자주 한국말을 안 해요.
I don’t speak Korean often because I’m shy.
한국말을 배워 본 적 없으니까 한국말을 못 해요.
I can’t speak Korean because I’ve never learned it.