-기 vs. -는 것 — when verbs become subjects and objects
Korean turns a verb into a noun-like 'thing' two ways. -기 sticks straight onto the stem to form a gerund: 배우다 → 배우기 ('learning'), 가다 → 가기 ('going'). -는 것 wraps the verb's modifier form around 것 ('thing'): 배우는 것 ('the act of learning'), 가는 것 ('the going'). The two often overlap, but each has distinct sweet spots: -기 is more abstract / lexical (and the only form for many fixed expressions), while -는 것 is more concrete / propositional ('the fact of doing X', 'the thing of doing X'). At B1, learn the patterns where each is required and where they trade. 한국어 배우기 어려워요 and 한국어를 배우는 것이 어려워요 both mean 'Learning Korean is hard,' but the first feels like a topic label and the second like a sentence subject.
-기 — the gerund / topic-label form
Add -기 straight to the stem: 먹다 → 먹기, 읽다 → 읽기, 공부하다 → 공부하기. Use it for: (1) abstract topics / activity names: 읽기 ('reading' as a skill), 쓰기 ('writing'), 듣기 ('listening'), 말하기 ('speaking') — exactly the four language-skill labels you see on Korean textbooks. (2) Pre-set lexical patterns (memorize these): -기 좋다 / 나쁘다 ('good/bad to do'), -기 쉽다 / 어렵다 ('easy/hard to do'), -기 시작하다 ('start to do'), -기 위해서 ('in order to'), -기 전에 ('before doing'). 이 음식은 만들기 쉬워요 = 'This food is easy to make.' (3) Casual subject-less topics: 운동하기 싫어요 = 'I don't feel like exercising.' Without an explicit subject or object marker, -기 has a label-like / heading feel.
-는 것 — the bound-noun form
Take the -는 modifier of an action verb (or -ㄴ/은 for adjectives) and add 것 ('thing'): 먹는 것 ('eating, the act of'), 가는 것 ('going'), 예쁜 것 ('the pretty one'). Use it for: (1) Subjects/objects of full sentences with markers: 운동하는 것이 좋아요 = 'Exercising is good.' (2) Reporting facts: 그가 한국 사람인 것을 몰랐어요 = 'I didn't know he was Korean (lit. the fact that he is Korean).' (3) 'The one(s) that...' / nominalized relative clauses: 민지가 만든 것 = 'the one Minji made.' 것 in speech often contracts to 거: 운동하는 거 좋아요, 이 거 = 'this thing'. With markers: 거 + 이 → 게, 거 + 을 → 걸. So 운동하는 게 좋아요 is the spoken form of 운동하는 것이 좋아요.
Where they're interchangeable (and where not)
When the gerund is the topic of a sentence, -기 + 어렵다/쉽다/좋다/싫다 and -는 것이 어렵다/쉽다/좋다/싫다 both work, with a register difference. 한국어 배우기 어려워요 (label-y, conversational headline) ≈ 한국어를 배우는 것이 어려워요 (full sentence with subject marker). The -는 것 version sounds slightly more deliberate/written. For fixed expressions like -기 시작하다 ('start to V'), -기 전에 ('before V-ing'), -기 위해서 ('in order to V'), -기 때문에 ('because of V-ing'), only -기 works — these are lexicalized. Conversely, when you need to embed a proposition ('the fact that...'), use -는 것: 그가 간 것을 알아요 = 'I know that he went' (NOT 그가 가기).
Idiomatic -기 expressions
These are worth memorizing as chunks: -기 시작하다 ('begin V-ing'): 비가 오기 시작했어요 = 'It started to rain.' -기 위해서 ('in order to V'): 건강해지기 위해서 운동해요 = 'I exercise in order to get healthy.' -기 때문에 ('because of V-ing'): 늦었기 때문에 미안해요 = 'I'm sorry because I was late' (note past tense possible). -기 전에 / -기 후에 are NOT a pair: 'before' is -기 전에, 'after' is -(으)ㄴ 후에 (NOT -기 후에). -기로 하다 ('decide to V'): 여행 가기로 했어요 = 'I decided to go on a trip.' -기 마련이다 ('it's only natural to V'): 실수하기 마련이에요 = 'mistakes happen / it's natural to make mistakes.'
When -는 것 is required
Three contexts demand -는 것 specifically: (1) embedded propositions under verbs like 알다 ('know'), 모르다 ('not know'), 생각하다 ('think'), 믿다 ('believe'): 그가 가는 것을 봤어요 = 'I saw him go' (lit. 'saw the going of him'). (2) 'The one(s)' / 'this one' with adjective + 것: 큰 것 = 'the big one', 빨간 것 = 'the red one'. (3) Expressing 'what': 내가 좋아하는 것 = 'what I like' (lit. 'the thing I like'). The English 'what' as a noun phrase is usually -는 것. Don't try -기 in any of these — it sounds like a typo.
-기 vs. -는 것 with adjectives
-기 can attach to adjectives too, but it's mostly limited to fixed patterns: 춥기 is rare; you'd say 추운 것 for 'the cold one' or 'being cold'. 추운 것이 싫어요 = 'I don't like it being cold.' For adjectives, default to -ㄴ/은 것. The few adjective -기 patterns you'll see: -기 쉽다 / 어렵다, -기 좋다 / 나쁘다 when describing how easy/good it is to be in a state — but these are usually built on action verbs, not adjectives. Rule of thumb: if you're not sure, -는 것 / -ㄴ/은 것 is a safer choice for adjectives, -기 for action-verb gerunds in fixed patterns.
