-고 있다 (in progress) vs. -아/어 있다 (resulting state)
English's 'I'm sitting' is ambiguous: it can mean 'in the act of sitting down' or 'currently in a seated position'. Korean splits these. -고 있다 = action in progress ('-ing'). 앉고 있어요 = 'I'm in the act of sitting down (lowering myself).' -아/어 있다 = resulting state after a one-shot action. 앉아 있어요 = 'I'm seated / sitting (in the chair).' Use -고 있다 with continuous activities (eat, study, watch). Use -아/어 있다 with change-of-position / change-of-state intransitive verbs: sit, stand, lie down, open, close, arrive, hang, gather. The action happens once; what continues is the state. Many beginners default to -고 있다 for everything English '-ing' would cover; learning -아/어 있다 is what makes you stop saying 앉고 있어요 when you mean 'I'm sitting'.
-고 있다 — ongoing action
Form: action verb stem + -고 있다. 먹다 → 먹고 있어요 = 'I'm eating.' 공부하다 → 공부하고 있어요 = 'I'm studying.' 읽다 → 읽고 있어요 = 'I'm reading.' Use for activities that continuously unfold through time. Past: 먹고 있었어요 = 'I was eating.' Future / probable: 먹고 있을 거예요 = 'I'll probably be eating.' Honorific: 계시다 replaces 있다: 읽고 계세요 = '(elder) is reading.' -고 있다 corresponds closely to English progressive '-ing': eat, drink, run, learn, watch, work.
-아/어 있다 — resulting state
Form: stem + -아/어 있다 (vowel rule same as everywhere): 앉다 → 앉아 있어요 ('is seated'), 서다 → 서 있어요 ('is standing'), 눕다 → 누워 있어요 ('is lying down', ㅂ-irregular: 누워). Use for intransitive change-of-position / change-of-state verbs where the action happens once and the state continues: 앉다 ('sit down'), 서다 ('stand up'), 눕다 ('lie down'), 열리다 ('open' [intr]), 닫히다 ('be closed'), 켜지다 ('be turned on'), 꺼지다 ('be turned off'), 떨어지다 ('be fallen / dropped'), 남다 ('remain'). 문이 열려 있어요 = 'The door is open.' 불이 켜져 있어요 = 'The light is on.'
Posture and location verbs
The classic -아/어 있다 group: posture verbs (앉아 있다 'is seated', 서 있다 'is standing', 누워 있다 'is lying down') and location/arrival verbs (가 있다 'has gone (and is there)', 와 있다 'has come (and is here)', 남아 있다 'remains', 살아 있다 'is alive'). Compare: 수민 씨가 가고 있어요 = 'Sumin is on her way going (in motion).' 수민 씨가 가 있어요 = 'Sumin has gone (she's already there).' 민지 씨가 와 있어요 = 'Minji is here (already arrived).' This 'has X-ed and is in that state' meaning is uniquely -아/어 있다's job.
Why some verbs only take one form
Transitive verbs generally don't take -아/어 있다 (which needs an intransitive subject in a result-state). You CAN'T say 책을 읽어 있어요 for 'I have read the book' — use 읽었어요 for past or 읽고 있어요 for ongoing. The closest to 'is currently read' is the lexical passive 읽혀 있다 or descriptive 읽혀져 있다. Continuous activities without a clear endpoint (eat, drink, study, run) don't take -아/어 있다 either: 먹어 있다 isn't a thing. Memorize the -아/어 있다 verbs as a closed list rather than trying to derive: 앉다, 서다, 눕다, 열리다, 닫히다, 켜지다, 꺼지다, 떨어지다, 남다, 살다, 죽다, 도착하다 (sometimes), 가다/오다.
The classic confusion: 입다, 쓰다, 신다
Korean clothing verbs are interesting: 입다 ('put on / wear'), 쓰다 ('wear on head, glasses'), 신다 ('wear shoes'). To say 'I'm putting on (in motion)': 입고 있어요 ('I'm in the process of putting on'). To say 'I'm wearing (already on)': 입고 있어요 ALSO works as a state — Korean lumps both together for clothing. So 빨간 셔츠를 입고 있어요 = 'I'm wearing a red shirt' OR 'I'm putting on a red shirt' — context tells you which (usually wearing). For unambiguous 'wearing/has on', some dialects use -고 있다 but written Korean sometimes prefers -(으)ㄴ 채로 or descriptive 입은. At B1, just remember: 입고 있어요 = 'I'm wearing' is fine and standard.
Practical pairings
Compare these pairs to feel the difference. (a) 앉고 있어요 = 'I'm in the act of sitting down (lowering)' vs. 앉아 있어요 = 'I'm seated (already down).' (b) 누워요 ('I lie down', simple action) vs. 누워 있어요 ('I'm lying down (in that position).') (c) 꽃이 피고 있어요 ('the flower is blooming, in process') vs. 꽃이 피어 있어요 ('the flower is in bloom (already open).') (d) 문이 열려요 ('the door opens', as it happens) vs. 문이 열려 있어요 ('the door is open.') When in doubt: if you can replace the English with 'is in [state]', you want -아/어 있다. If you can replace with 'is X-ing (continuous activity)', you want -고 있다.
