Saying no: 안 and 아니에요

안 goes directly before the verb

가요 — 안 가요. One small word in front of the verb turns “go” into “don’t go”. The same in a longer sentence: 커피를 안 마셔요, 고기를 안 먹어요.

안 is written as a separate word and stands directly before the verb. The object or the place stays where it was, in front of 안. One form covers both “I don’t drink” and “I’m not drinking”.

With 공부해요 and 일해요, 안 goes in the middle

공부해요 becomes 공부 안 해요, and 일해요 becomes 일 안 해요. The rule has not changed: 안 stands directly before the verb, and here the verb is 해요. So 안 lands between the noun and 해요.

One word behaves differently. 좋아해요 looks like this family but is a single word that cannot be split: 안 좋아해요.

“Is not” is a different word: 아니에요

저는 학생이에요 — 저는 학생이 아니에요. To say what someone or something is not, the noun takes 이 or 가 and is followed by 아니에요. 이거 물이 아니에요: this is not water.

안 cannot be used here. It works with verbs, and 이에요 / 예요 is not one of them. You already know 아니에요 as the reply to thanks; literally that reply says “it is not (so)”.

이 after a final consonant (학생이 아니에요), 가 after a vowel (커피가 아니에요).

Not being is not the same as not having

이거 커피가 아니에요 — this is not coffee; it is something else. 커피가 없어요 — there is no coffee; none exists here, or I have none.

Both sentences have 커피가, so look at the last word. 아니에요 denies what a thing is. 없어요 denies that it is there. English uses “not” or “no” for both and for 안 as well; Korean keeps three separate tools.