-아/어지다 — the suffix that flips state into change
Stick -아/어지다 to a Korean stem and you get one of two related meanings, depending on the verb type. With descriptive verbs (adjectives) you get a state change: 좋다 ('be good') → 좋아지다 ('become good / get better'). With action verbs you get a productive passive: 만들다 ('make') → 만들어지다 ('be made'). The vowel rule is the same as everywhere else in Korean: stems whose final vowel is ㅏ or ㅗ take -아지다; everything else takes -어지다; 하다 verbs become -해지다. Conjugate the -지다 part normally — it's a regular ㅣ-stem verb. So 좋아지다 → 좋아져요 (becomes good), 좋아졌어요 (became good), 좋아질 거예요 (will become good).
With adjectives — 'becoming X'
Korean adjectives describe states (크다 = 'be big'), not actions. To say something becomes big, attach -아/어지다 to the stem. 크다 → 커지다 ('get bigger'), 작다 → 작아지다 ('get smaller'), 예쁘다 → 예뻐지다 ('become pretty'), 바쁘다 → 바빠지다 ('get busy'). For 하다 adjectives: 건강하다 → 건강해지다 ('get healthy'), 조용하다 → 조용해지다 ('get quiet'). Past tense is the natural fit for completed change: 날씨가 추워졌어요 = 'The weather got cold.' 얼굴이 빨개졌어요 = 'Her face got red.' Use present -아/어져요 for ongoing change: 점점 좋아져요 = 'It's gradually getting better.'
With action verbs — productive passive
Korean has 'lexical' passives baked into specific verbs (보다 → 보이다 = 'be seen'), but you can also build a passive on almost any action verb with -아/어지다. 만들다 ('make') → 만들어지다 ('be made'), 쓰다 ('write') → 써지다 ('be written'), 느끼다 ('feel') → 느껴지다 ('be felt'). 이 책은 한국어로 쓰여졌어요 = 'This book was written in Korean.' (You'll also see 쓰여 있다 = 'is written/posted'.) The -아/어지다 passive often emphasizes the process or result, not the agent — that's why it pairs naturally with subjects like 'the cake', 'the door', 'the feeling'. Compare: 문이 열려요 (lexical, 'the door opens') vs. 문이 열어져요 (productive, 'the door gets opened') — both exist; the productive form sometimes carries a 'comes to be opened' nuance.
Vowel rules and irregulars
Stem ends in ㅏ/ㅗ → -아지다: 좋다 → 좋아지다, 작다 → 작아지다. Stem ends in any other vowel → -어지다: 싫다 → 싫어지다 ('come to dislike'), 늦다 → 늦어지다 ('get late'). 하다 → 해지다: 편하다 → 편해지다 ('get comfortable'). ㅂ-irregular adjectives turn ㅂ into 우 before -어지다: 춥다 → 추워지다 ('get cold'), 덥다 → 더워지다 ('get hot'), 쉽다 → 쉬워지다 ('get easier'). ㅡ-irregular: drop ㅡ and check the previous vowel: 바쁘다 → 바빠지다 (previous vowel ㅏ), 예쁘다 → 예뻐지다 (previous vowel ㅖ, treat as not ㅏ/ㅗ).
When NOT to use -아/어지다
Already-action verbs that mean 'become' on their own don't need it: 되다 ('become', 'turn into') is the verb of choice for nouns: 의사가 되다 = 'become a doctor', not 의사가 돼지다. Use -아/어지다 mainly for adjective stems plus a handful of action verbs. Also, some adjectives have lexicalized variants: 많아지다 ('increase') is fine, but Koreans often just say 늘다 ('grow / increase'). Likewise 적어지다 ('decrease') competes with 줄다. Both forms are correct; -아/어지다 is the safe productive option when you're not sure if a more idiomatic verb exists.
Tense and chaining
-아/어지다 is itself a verb, so it conjugates fully. Past: 좋아졌어요 ('became good'). Future / supposition: 좋아질 거예요 ('it'll get better'). Negation: 안 좋아져요 ('isn't getting better'), or 좋아지지 않아요 (more formal). You can chain it with other endings: 추워지면 ('if it gets cold'), 추워져서 ('because it got cold'), 추워질 때 ('when it gets cold'). 점점 ('gradually') and 갑자기 ('suddenly') pair naturally: 갑자기 추워졌어요 = 'It suddenly got cold.'
