-는 / -ㄴ/은 / -ㄹ/을 — three tenses for relative clauses
Korean doesn't have words like 'who' or 'that' for relative clauses. Instead, the whole verb phrase comes BEFORE the noun, and the verb takes a special modifier ending that also marks tense. Three core endings to learn: -는 (present, action verb): 먹는 사람 = 'a person who eats / is eating'. -ㄴ/은 (past for action verbs, also general for adjectives): 먹은 사람 = 'a person who ate'. -ㄹ/을 (future or 'about to'): 먹을 사람 = 'a person who will eat / is going to eat'. The order is English's reverse: where English says 'the book that I read', Korean says 내가 읽은 책 ('I-read book'). Learn the three endings, learn how they attach to vowel-final vs. consonant-final stems, and you can describe any noun with any verb.
-는 — present action ('who is V-ing')
For action verbs only. Attach -는 straight to the stem regardless of whether it ends in a vowel or consonant. 먹다 → 먹는, 가다 → 가는, 공부하다 → 공부하는. 먹는 사람 = 'a person who eats / is eating'. 한국어를 공부하는 학생 = 'a student who studies Korean'. 재미있다 ('be interesting') is grammatically an action-like adjective and takes -는: 재미있는 영화 = 'an interesting movie'. Same with 맛있다, 있다, 없다: 돈이 없는 사람 = 'a person who has no money'.
-ㄴ/은 — past action / general adjective
Two jobs share one ending. Past action: attach -ㄴ after a vowel-stem, -은 after a consonant-stem: 가다 → 간, 먹다 → 먹은. 어제 만난 사람 = 'the person I met yesterday'. General adjective: the same -ㄴ/은 turns adjectives into noun-modifiers (NO tense — just 'the X one'). 크다 → 큰, 작다 → 작은, 예쁘다 → 예쁜. 큰 집 = 'a big house', 작은 가방 = 'a small bag'. ㅂ-irregular adjectives soften: 춥다 → 추운, 덥다 → 더운, 쉽다 → 쉬운. 하다 adjectives: 조용하다 → 조용한.
-ㄹ/을 — future / prospective
-ㄹ after a vowel stem, -을 after a consonant stem: 가다 → 갈, 먹다 → 먹을. 내일 만날 사람 = 'the person I'll meet tomorrow'. 읽을 책 = 'a book to read / a book I'll read'. This ending often also covers 'one to / one for V-ing': 마실 물 = 'water to drink', 쓸 펜 = 'a pen to write with'. Combined with 거 ('thing') it forms the future -ㄹ 거예요: 갈 거예요 = 'will go' — that's actually this same modifier attached to 거.
Word order — head noun comes last
English puts modifiers AFTER the noun ('the book that I read'); Korean puts them all BEFORE: 내가 읽은 책 ('I-[past]-read book'). The subject of the inner clause stays inside, but the noun being described — the head noun — sits at the end. 민지가 만든 케이크 = 'the cake (that) Minji made'. 우리가 가는 식당 = 'the restaurant we're going to'. The 그가/그녀가 subject (he/she) often gets dropped if obvious from context. Note that the inner subject is usually marked with -가/이 (or -는/은 for contrast), not -는 with the topic feel of a main sentence.
Past for adjectives: -았/었던 vs. -ㄴ/은
Adjectives use -ㄴ/은 for general/timeless modification (큰 집 = 'a big house, currently'). For 'was X then' — explicit past state — Korean has the special -았/었던 ending: 컸던 집 = 'a house that was big (no longer or contrast intended)'. Action verbs can also use -았/었던 for a 'used-to' nuance: 자주 갔던 카페 = 'the cafe I used to go to often'. Compare with 간 ('went, completed once') and 가는 ('go, currently'). At B1, -ㄴ/은 for past action and -는 for present action handle most cases; -았/었던 is the more nuanced 'recollected' version.
Adjective-like 있다 / 없다
있다 and 없다 look like adjectives in meaning ('exist / not exist') but behave like action verbs in modifier endings. They take -는, never -은: 돈이 있는 사람 = 'a person who has money', 시간이 없는 학생 = 'a student who has no time'. The same logic extends to compounds with -있다 / -없다: 재미있는 ('interesting'), 재미없는 ('boring'), 맛있는 ('delicious'), 맛없는 ('not tasty').
