Not: 안 before the verb

The simplest negative is 안, placed directly before the verb or adjective, with a space: 가요 → 안 가요, 비싸요 → 안 비싸요, 먹었어요 → 안 먹었어요. The verb itself does not change: tense and politeness stay where they were. There is no helper like "do" or "be" to build, and nothing else in the sentence moves.

안 means "not" as a matter of fact or of choice: it does not happen, I do not do it. 오늘 학교에 안 가요 is "I'm not going to school today", and the listener will understand that there is no school or that you have decided not to go. If you would like to go and cannot, that is 못, the next topic.

With a verb made of a noun and 하다, 안 goes before 하다, not before the whole verb: 공부해요 → 공부 안 해요, 운동했어요 → 운동 안 했어요. Putting it in front, 안 공부해요, is the classic beginner's mistake. Adjectives in 하다 are different, because their 하다 does not mean "do": 안 피곤해요, 안 깨끗해요. So are verbs like 좋아하다 whose first part is not a noun on its own: 안 좋아해요.

안 belongs to everyday speech and suits short, common words; in formal speech and in writing the long form -지 않다 is preferred. It is not used with 있다 in the sense of "there is" or "have", with 알다, or with the copula, which have their own negative words (없다, 모르다, 아니다). In a question, 안 often makes a gentle invitation or a check: 안 가요?, "aren't you going?".