It's very tricky since Korean language doesn't distinguish between "have to" and "shall". I have to go now: 지금 가야해요. I should go now: 지금 가는게 좋겠어요. (I suppose it would be wise to go now) "should not" can be translated to "안 돼" (not allowed to) I should not talk to you since I have to study now: 지금 공부해야해서 당신과 얘기하면 안 돼요.