In short

A Korean clause can do two grown-up jobs. Put a modifier ending on the verb and the whole clause becomes an adjective in front of a noun - a relative clause, whose own subject takes 이/가 (never 은/는): 제가 어제 산 책 (jega eoje san chaek, the book I bought yesterday). Or turn the clause into a noun by adding -기 or -는 것: 수영하기 (suyeonghagi, swimming), 자는 것 (janeun geot, sleeping).

The rule

Step 1 - A relative clause is a modifier clause with its own subject

Lesson 24 gave you the modifier endings -는 / -(으)ㄴ / -(으)ㄹ and the bound nouns. A relative clause is the next step up: a full clause - subject, object, adverb and all - turned into a modifier and set in front of the head noun. Because Korean has no who/which/that, the ending does all the linking: Clause + modifier ending + Noun.

The subject inside the modifier clause takes the subject particle 이/가, never the topic 은/는 - 은/는 belongs to the main clause. So the book I bought is 제가 산 책 (jega san chaek), with 제가, not *저는.

Step 2 - The head noun fills a gap in the clause

The noun out front is understood as playing a role - subject, object, or place - back inside the clause, where there is a silent “gap”. The same verb points either way depending on which slot is empty:

• 책을 읽는 사람 (chaegeul ingneun saram) - the person who reads a book (head = subject; 책을 stays)
• 제가 읽는 책 (jega ingneun chaek) - the book I read (head = object; 제가 stays)
• 제가 사는 곳 (jega saneun got) - the place where I live (head = an adverbial “where”)

Step 3 - Tense lives on the modifier ending (recap of lesson 24)

Which ending you choose still fixes the time of the embedded action: present -는, past -(으)ㄴ, prospective -(으)ㄹ. 어제 (yesterday) forces past 산; 내일 (tomorrow) forces prospective 살.

Step 4 - Modifiers can be long, and can stack

A modifier clause can be as long as you like - just keep everything before the ending: 제가 어제 도서관에서 빌린 책 (jega eoje doseogwaneseo billin chaek, the book I borrowed from the library yesterday). You can also stack two modifiers on one noun: 제가 좋아하는 조용한 카페 (jega joahaneun joyonghan kape, a quiet cafe that I like).

Step 5 - Nominalization: making the clause into a noun

To use a clause where a noun is needed (as a subject or object), nominalize it two ways - and if the nominalized clause has a subject of its own, that subject takes 이/가 and never 은/는, exactly as inside a relative clause:

• -기 attaches straight to the stem (하다 -> 하기, 먹다 -> 먹기, 살다 -> 살기) and names the activity in the abstract. It is the form required or preferred by certain predicates - 좋아하다, 싫어하다, 시작하다, 쉽다, 어렵다, 힘들다, 바라다 - and by fixed frames like -기 전에 (before -ing).
• -는 것 is a modifier ending plus the bound noun 것 (thing/act/fact). It names the act or fact as a concrete thing, carries tense through the ending (-(으)ㄴ 것 / -는 것 / -(으)ㄹ 것), and pairs with 좋다, 좋아하다, 알다, 보다. Because 것 is a noun, it takes ordinary particles - and they contract in speech: 것이 -> 게, 것을 -> 걸, 것은 -> 건. The everyday advice pattern is -는 게 좋아요 (it’s better to …).

Step 6 - Which one? (and one deferral)

Reach for -기 inside its fixed lexical frames (see the selection table); reach for -는 것 everywhere else, especially in speech and whenever tense matters. A third nominalizer, the bookish -(으)ㅁ, waits for lesson 58.

Forms & patterns

Where the head noun sits in the clause (the “gap”):

Tense on the modifier ending (recap - full paradigm in lesson 24):

-기 vs -는 것 - which nominalizer:

것 + particle contractions (spoken, but written too):

Examples

Common mistakes

• 어제 저는 산 책이 좋아요 (eoje jeoneun san chaegi joayo) -> 어제 제가 산 책이 좋아요 (eoje jega san chaegi joayo) - the subject inside a relative clause takes 이/가, so 저 becomes 제가; 은/는 marks the main topic and cannot live inside the modifier.
• 제가 어제 사는 책 (jega eoje saneun chaek) -> 제가 어제 산 책 (jega eoje san chaek) - 어제 is past, so the modifier ending must be past -(으)ㄴ (산), not present -는. The ending carries the tense.
• 저는 운동하는 게 좋아해요 (jeoneun undonghaneun ge joahaeyo) -> 저는 운동하는 걸 좋아해요 (jeoneun undonghaneun geol joahaeyo) - 좋아하다 is transitive and needs the object 것을 -> 걸; only the descriptive 좋다 takes 것이 -> 게 (운동하는 게 좋아요).
• 운동하는게 좋아요 (undonghaneunge joayo) -> 운동하는 게 좋아요 (undonghaneun ge joayo) - 것/게 is a bound noun, so it keeps a space before it even after contracting.
• 자는 것 전에 (janeun geot jeone) -> 자기 전에 (jagi jeone) - 전에 pairs only with the -기 nominalizer, never with -는 것.

Tips

• Two silent rules. Inside an embedded clause - relative or nominalized - the subject is 이/가; and in front of the head noun there is nothing but the ending. If you find yourself reaching for 은/는 or a “that”, stop - Korean uses neither.
• -기 is the “fixed-frame” nominalizer, -는 것 is the everyday one. When a predicate or set phrase demands -기 (좋아하다, 시작하다, -기 전에), use it; otherwise, especially in speech, -는 것 / -는 게 is almost always natural.
• 것 always keeps its space, even shrunk. 게, 걸, 건 are just 것 + a particle squeezed together (것이, 것을, 것은). Say the full form once and the space stays obvious.
• 좋다 vs 좋아하다 decide your particle. 좋다 (to be good) is descriptive -> -는 게 (좋아요); 좋아하다 (to like) is transitive -> -는 걸 (좋아해요). Same clause, different particle.