I can't do it: - (난 그건) 못해 / (그건) 못하겠어 / 하지 못하겠어 / 할 수가 없어. I can't eat it: - (난 그건) 못 먹어 / (이건) 못 먹겠어 / 먹지 못하겠어 / 먹을 수가 없어. I can't go: - 못 가 / 못 가겠어 / 가지 못하겠어 / 갈 수가 없어. 못 is used for "can't" (ability, circumstance), whereas 안 is for "won't" (willingness)

Korean conveys more of its meaning through grammar rather than words. So there's no one way just to tell you the words "I can not". There's a few grammar structures that you could use depending on the context and exact meaning.