In short

Korean negates a sentence in two ways: put the little adverb 안 (an, not) or 못 (mot, can’t) in front of the predicate, or attach the long ending -지 않다 / -지 못하다 to the stem. 안 denies; 못 says you were unable. Three verbs refuse the negative adverbs and use a separate negative word instead: 있다 -> 없다, 알다 -> 모르다, 이다 -> 아니다.

The rule

Step 1 - Two negatives, and they mean different things

안 is plain denial: it doesn’t happen, it isn’t so, or I choose not to. 못 says the action was blocked - by lack of ability, by circumstances, by someone else.

• 안 가요. (An gayo.) - I’m not going. (I don’t want to, I have no reason to.)
• 못 가요. (Mot gayo.) - I can’t go. (Something is stopping me.)

Because 못 is about ability, it attaches only to 동사 (action verbs). An adjective describes a state, and a state cannot be “unable”, so *못 바빠요 is impossible - the negative of 바빠요 is 안 바빠요 (an bappayo, isn’t busy).

Step 2 - Short form: a free adverb standing before the predicate

안 / 못 + [predicate], with a space.

They stand immediately before the verb - after the object, after the place, after everything else, because Korean is verb-final:

Object + 을/를 + 안 + Verb - 저는 아침을 안 먹어요. (Jeoneun achimeul an meogeoyo.) - I don’t eat breakfast.

The space matters. 안 가다 is not to go, but 안가 written solid is a different word altogether, and 못 하다 (to be unable to do it) is not 못하다 (to be bad at something) - see the Tips.

Step 3 - Long form: an ending stacked on the stem

Verb or adjective stem + -지 않다 / Verb stem + -지 못하다

This one is a real ending, so it goes on the stem, and then 않다/못하다 conjugates as usual: 않다 -> 않아요 (anayo), 못하다 -> 못해요 (motaeyo).

가다 -> 가- -> 가지 않아요 (gaji anayo); 먹다 -> 먹- -> 먹지 못해요 (meokji motaeyo).

Two things make -지 unusually easy. It has no allomorphs - unlike 은/는, 이/가 or -(으)면, it attaches in one single shape whether the stem ends in a vowel or a 받침. And because it starts with a consonant, it never triggers an irregular stem: 덥다 -> 덥지 않아요 (deopji anayo), not *더우지 않아요.

Spacing: 않아요 and 못해요 are separate auxiliary words, so 하지 않아요 takes a space. *하지않아요 is a spelling error.

The long form is not more polite - it is more written and more emphatic. In ordinary speech the short form is far more common.

Step 4 - 하다 verbs split in the short form only

A noun + 하다 verb like 공부하다 is one word, but 안 and 못 wedge themselves in between the noun and 하다:

• 공부 안 해요. (Gongbu an haeyo.) - I don’t study. Never *안 공부해요.
• 공부 못 해요. (Gongbu mot haeyo.) - I can’t study.

The long form does not split, because it is an ending on the whole stem: 공부하지 않아요 (gongbuhaji anayo).

Two groups look like 하다 verbs but do not split:

• 하다-adjectives: only a 동사 splits, so 안 stays in front of the whole word - 안 피곤해요 (an pigonhaeyo, isn’t tired), not *피곤 안 해요. (피곤 is a perfectly good noun; what blocks the split is that 피곤하다 is a 형용사.)
• Lexicalized verbs whose 하다 sits on a verb form rather than a noun: 안 좋아해요 (an joahaeyo, doesn’t like), 안 싫어해요 (an sireohaeyo, doesn’t dislike).

Step 5 - Three verbs have a suppletive negative

Some words own a separate negative word, and the negative adverbs are simply ungrammatical with them.

• 있다 -> 없다: *안 있어요 is wrong; say 없어요 (eopseoyo). See lesson 6.
• 알다 -> 모르다: *안 알아요 is wrong; say 몰라요 (mollayo, 르-irregular).
• 이다 -> 아니다: *학생이 안 이에요 is wrong; say 학생이 아니에요 (haksaengi anieyo), with 이/가 on the noun. See lesson 2.

The block is on the short adverb. The long ending is still possible in careful writing (알지 못해요, 있지 않아요), but as a beginner use the suppletive word.

Step 6 - 못 is spelled 못 and pronounced [몯]

The 받침 ㅅ in 못 is never an [s]. It neutralises to a plain [t] stop, and then reacts to whatever follows - this is why 못 sounds different in almost every sentence while the spelling never changes. The full set of rules lives in the reading and pronunciation guide, in its lesson on the sound changes that the spelling hides; the table below has the four cases you need now.

Step 7 - Adverbs that require a negative

A small set of adverbs and adverbial phrases is negative-polarity: they license, and demand, a negative predicate. Saying them with a positive verb is a real error, not a stylistic slip.

별로 (byeollo, not particularly), 전혀 (jeonhyeo, not at all), 하나도 (hanado, not one bit - literally 하나 + the particle 도, see lesson 15).

별로 안 좋아해요. (Byeollo an joahaeyo.) - I don’t like it much. But *별로 좋아해요 is wrong - there is no positive 별로.

아직 (ajik) is not one of these. Paired with a negative it means not yet (아직 안 왔어요. Ajik an wasseoyo. - hasn’t come yet), but with a positive predicate it simply means still (아직 자요. Ajik jayo. - is still asleep), so on its own it does not demand a negative.

Forms & patterns

안 vs 못 - the core contrast:

Short form vs long form - same meaning, different register:

-지 않다 attaches in one shape - no allomorphs:

The three suppletive negatives:

How 못 is pronounced:

Revised Romanization writes assimilation and aspiration but not tensing, so 못 가요 is romanized mot gayo even though the ㄱ is tense.

Adverbs that demand a negative:

Examples

Common mistakes

• 안 공부해요. (An gongbuhaeyo.) -> 공부 안 해요. (Gongbu an haeyo.) - In the short form, 안 splits a noun + 하다 verb and stands just before 하다. Only the long form keeps the word whole: 공부하지 않아요.
• 저는 안 알아요. (Jeoneun an arayo.) -> 저는 몰라요. (Jeoneun mollayo.) - 알다 has its own negative word, 모르다. The same block applies to 있다, whose negative is 없다.
• 어제 안 잤어요. (Eoje an jasseoyo.) -> 어제 못 잤어요. (Eoje mot jasseoyo.) - If sleep would not come, that is inability: 못. 안 잤어요 says you deliberately stayed up.
• 가지않아요. (Gajianayo.) -> 가지 않아요. (Gaji anayo.) - 않아요 is an auxiliary verb, not a suffix, so 띄어쓰기 requires the space. Same for 하지 못해요.
• 오늘은 못 바빠요. (Oneureun mot bappayo.) -> 오늘은 안 바빠요. (Oneureun an bappayo.) - 못 negates only action verbs. A state cannot be “unable”, so adjectives always take 안.

Tips

• 안 = won’t, 못 = can’t. Ask yourself whether the thing failed to happen because of a decision (안) or because of an obstacle (못). English blurs this - I didn’t go covers both - so the choice has to be made consciously every time.
• The space is a different word. 못 하다 (mot hada) is couldn’t do it; 못하다 (motada) written solid is to be bad at it. So 수영을 못해요 = I can’t swim / I’m a poor swimmer, while 숙제를 못 했어요 = I didn’t manage to do the homework. Likewise 잘 못해요 (I’m not good at it) vs 잘 못 해요 (I couldn’t do it properly).
• Speak short, write long. Learners often reach for -지 않다 because it looks more like a grammar rule. Native speech reaches for 안 and 못 first. Default to the short form and keep the long one for writing and for stressing the denial.
• 별로 and 전혀 are half a negative. They cannot stand alone the way English hardly or barely can - the negative predicate must still be there. If you say 별로, your sentence has already promised an 안, a 못 or a 없어요 at the end.