Choosing -기 vs -(으)ㅁ vs -는 것

To use a Korean verb where a noun belongs — as a subject, an object, the thing you like or find hard — you have to turn the verb into a noun. Korean gives you three tools for that job: -기, -(으)ㅁ, and -는 것. English collapses all three into one flexible machine (the -ing form: "swimming is fun," "I know his leaving," "I saw him leave"), so learners arrive expecting a single choice and instead face a fork with three paths. The good news is that the fork is not random. Once you learn to read a clause for two things — its aspect (is this an activity, or a settled fact?) and its register (are you talking, or writing a report?) — the right nominalizer usually selects itself.

The one-line summary

• -기 = an unrealized activity — a doing, viewed as a process, especially one you can be good at, bad at, fond of, or that a fixed frame demands. Glued to feeling and ease/difficulty predicates.
• -(으)ㅁ = a fixed fact or state, packaged as a compact noun. Formal, written, and often lexicalized.
• -는 것 = the general-purpose, spoken, concrete "the fact/act that." Tense-flexible and by far the most common in conversation.

-기: the activity nominalizer

-기 takes a verb and freezes it as an activity, an unrealized doing viewed as a whole. Its natural home is right in front of a predicate that evaluates that activity — how fun, easy, hard, good, or unpleasant it is. Think of the predicates 재미있다 (fun), 쉽다 (easy), 어렵다 (hard), 좋다 (good/nice), 싫다 (unpleasant): these want an activity to weigh, and an activity is exactly what -기 delivers.

The second and third sentences are set collocations: 읽기(가) 쉽다, 끊기(가) 어렵다. In these fixed frames the activity reading is baked in, and swapping in -는 것 sounds off (more on that in the mistakes section). -기 is also the form demanded by a family of grammatical frames you meet everywhere: -기 전에 (before doing), -기 때문에 (because), -기로 하다 (decide to), -기 위해 (in order to). These frames don't give you a choice — they lexically require -기.

-(으)ㅁ: the fixed-fact nominalizer

-(으)ㅁ packs a whole proposition into a tight, noun-shaped bundle: 있음 (the fact of existing / "in stock"), 없음 ("none"), 믿음 (belief), 죽음 (death). Where -기 views a doing as an ongoing activity, -(으)ㅁ views it as an accomplished, established fact — something you could assert, prove, or record. That difference in aspect is why -(으)ㅁ gravitates to formal, written contexts: the complements of 밝히다 (reveal), 증명하다 (prove), 알다 (know/realize), 확인하다 (confirm), and the clipped nominal style of memos, forms, and dictionary entries.

Notice the register the second example lives in: a sticky note, a sign, a status field. That clipped -(으)ㅁ ending — 없음, 재고 있음, 확인함 — is a genuine written register, not something you say out loud at lunch. Many of these forms have also fully lexicalized into ordinary dictionary nouns (믿음 "faith," 웃음 "laughter," 꿈 "dream"); at that point they're just nouns and no longer feel like a live nominalization at all.

-는 것: the everything-else nominalizer

-는 것 is the workhorse. Literally "the thing/fact that (one) does," it nominalizes a full clause — complete with its own tense marking on the verb — and it is overwhelmingly the form of spoken and everyday Korean. Where -(으)ㅁ freezes a bare proposition, -는 것 keeps the clause alive and flexible: you can mark it present (-는 것), past (-은 것), or prospective (-을 것), and it slots comfortably into any register from casual chat to careful prose.

In speech 것을 routinely contracts to 걸, 것이 to 게, 것은 to 건 — 것 is so common that it wears down. If you catch yourself reaching for -(으)ㅁ in conversation, this is almost always the form you actually want instead.

The register spread: the same idea, two ways

The clearest way to feel the difference is to say the same thing in two registers. Compare the courtroom-formal -(으)ㅁ against its neutral, sayable twin built on -다는 것 (the fact-that complement, covered in depth on its own -다는 것 page):

Both are correct Korean. The first belongs in a legal brief or an academic paper; the second is what a person says. Choosing -(으)ㅁ in a chat with a friend isn't wrong the way a botched conjugation is wrong — it just lands like saying "I hereby acknowledge his correctness" at a dinner table. That mismatch of register, not grammar, is what usually gives learners away.

Where the choice is actually forced

Most of the time two options would be understood and only register separates them. But a few slots are genuinely fixed, and these are worth memorizing:

• Set -기 collocations and frames take -기 only: 읽기(가) 쉽다, 하기(가) 힘들다, and every -기 전에 / -기 위해 / -기로 하다 frame. You cannot substitute -는 것 into the idiom.
• The clipped nominal register (signs, memos, forms) takes -(으)ㅁ only: a "no vacancy" sign reads 빈방 없음, never 빈방 없는 것.
• Fact-complements of 알다/모르다 in speech overwhelmingly take -는 것 (or -는 줄), not -기: -기 does not report facts, only activities.

How this differs from English

English gives you one gerund — quitting is hard, I proved his being right, I saw him leaving — and lets context sort out the nuance. Korean front-loads that nuance into the choice of nominalizer, so you commit to an interpretation before the sentence is even finished. The upside is precision: a Korean listener knows from the ending alone whether you mean an activity (-기), a documented fact (-(으)ㅁ), or a live, tensed event (-는 것). The cost is that you can't stay neutral the way English lets you — you have to decide. When you genuinely can't, default to -는 것 in speech; it is the least marked option and almost never sounds wrong.

Common Mistakes

1. Reaching for -(으)ㅁ in casual conversation. It's grammatical but reads like a legal filing.

2. Forcing -는 것 into a fixed -기 collocation. The idiom demands -기.

3. Using -기 to report a fact. -기 packages activities, not the "the fact that…" complements of 알다/모르다.

4. Trying to give -기 a tense. -기 has no tense of its own; put the tense on a -는 것 clause instead.

Key Takeaways

• Sort by aspect + register: unrealized activity → -기; established/written fact → -(으)ㅁ; everything else, especially spoken → -는 것.
• -기 is obligatory in set frames (-기 전에, -기 때문에, -기로 하다) and with ease/difficulty/feeling predicates (쉽다, 어렵다, 좋다, 싫다).
• -(으)ㅁ is formal, written, and clipped-nominal (없음, 증명했음); using it in chat sounds like a legal document.
• -는 것 is the safe default in conversation, keeps its tense (-는/-은/-을 것), and contracts to 걸/게/건.
• The wrong choice is usually a register miss, not a grammar error — but knowing the forced slots (idioms → -기, signs → -(으)ㅁ) keeps you from sounding off.