Korean has no separate word for and, so or because between clauses — it glues an ending onto the first verb. -고 = and (independent actions). -아서/어서 = and then (first sets up second) or so (natural cause). -(으)니까 = because (your reasoning) — the only one allowed before a request. -지만 = but. Two rules decide most cases: -아서 never takes past tense, and never comes before -세요 or -(으)ㅂ시다.
Beginners speak Korean in short, separate sentences: 비가 와요. 늦었어요. 미안해요. It is correct, and it sounds like a telegram. The moment you can glue those sentences together, your Korean starts sounding like a person talking — and that takes just four endings.
And — lists and independent steps
The simplest glue: add -고 to any verb or adjective stem. No vowel rules, no irregulars. Tense is usually marked only on the last verb.
Remember: between nouns, and is a particle (빵하고 우유, 빵과 우유) — -고 is only for verbs and adjectives.
And then — when the first action sets up the second
Use -아서/어서 when the second action depends on the first: you go somewhere and do something there, you make something and do something with it, you meet someone and do something together. Formed like the polite present: 가요 → 가서, 만들어요 → 만들어서, 해요 → 해서.
The minimal pair that shows the difference
-고 → two separate events
친구를 만나고 영화를 봤어요.chingureul mannago yeonghwareul bwasseoyoI met a friend, and (later) watched a film.
Maybe alone, maybe with someone else.
-아서 → one connected event
친구를 만나서 영화를 봤어요.chingureul mannaseo yeonghwareul bwasseoyoI met a friend and we watched a film.
The meeting led into the film — together.
So / because: -아서 vs -(으)니까
Both endings can mean because, and this is where most learners guess. There is a clear rule, and one strong tendency.
A natural cause — neutral, factual
My reasoning — and the only choice before a request
After a consonant add -으니까 (먹으니까); after a vowel -니까 (오니까).
✗ 비가 와서 우산을 가져가세요.
✓ 비가 오니까 우산을 가져가세요.
✗ 어제 비가 왔어서 늦었어요.
✓ 어제 비가 와서 늦었어요.
-(으)니까 and -지만, by contrast, can carry past tense: 어제 많이 먹었으니까 오늘은 조금만 먹을게요 — I ate a lot yesterday, so today I'll eat just a little.
Apologies and thanks always use -아서
Natural
늦어서 죄송해요.neujeoseo joesonghaeyoSorry for being late.
도와줘서 고마워요.dowajwoseo gomawoyo
Thanks for helping.
Sounds like an excuse
늦으니까 죄송해요.Because -(으)니까 presents your argument, it turns an apology into self-justification. Keep -니까 for reasons you are putting forward, not ones you are apologising for.But
At the start of a new sentence use the standalone word 하지만 (neutral) or 그런데 (conversational). The softer, more open-ended -는데 is covered in Korean sentence endings.
The decision tree
| Ask | Use | Example |
|---|---|---|
| Is it a contrast? | -지만 | 싸지만 좋아요 |
| Is it a reason + request / suggestion? | -(으)니까 | 추우니까 문 닫아 주세요 |
| Is it an apology or thanks? | -아서/어서 | 늦어서 죄송해요 |
| Is it any other reason? | -아서/어서 (neutral) | 아파서 못 갔어요 |
| Does action 1 set up action 2? | -아서/어서 | 가서 봐요 |
| Otherwise — just “and” | -고 | 씻고 자요 |
Check yourself
Pick the ending that fits.
Reason + request (-세요) → -(으)니까. (추워서 is not allowed here.)
Apologies use -아서/어서.
-아서 never takes past tense: 와서.
The buying happened at the market → 가서.
Contrast → -지만.
Two independent steps → -고.
From telegram Korean to real sentences
Hanguljo takes you from Foundation lessons (Hangul, stroke order, native audio) through the TOPIK 1 to 6 lessons — vocabulary, grammar and particles — with Hanja roots and memory hooks for Sino-Korean words and a 7-stage SRS engine timing every review. Practise speaking in the conversation track with shadowing, train your ear with dictation, learn from any Korean video you paste in, read signs with the on-device OCR scanner, and keep going all day with home, lock screen and quiz widgets. Calm by design.
Learn Korean with Hanguljo - FreeQuick reference
- -고
- And — lists and independent steps. 싸고 맛있어요. 먹고 마셨어요.
- -아서/어서 (sequence)
- And then — first action sets up the second: 가서 공부했어요, 만나서 봤어요.
- -아서/어서 (reason)
- So — natural cause: 비가 와서 늦었어요. Apologies and thanks: 늦어서 죄송해요. No past tense, no commands after it.
- -(으)니까
- Because — your reasoning. Required before -세요, -(으)ㅂ시다, -(으)ㄹ까요: 비가 오니까 우산 가져가세요.
- -지만
- But: 어렵지만 재미있어요. Sentence-initial: 하지만, 그런데.
Frequently asked questions
What is the difference between -고 and -아서?
-고 lists independent actions; -아서 links actions where the first sets up the second. 만나고 봤어요 vs 만나서 봤어요.
-아서 or -니까 for “because”?
-아서 for neutral causes; -(으)니까 for your reasoning and always before a request or suggestion.
Why 늦어서 죄송해요?
Apologies and thanks use -아서/어서; -니까 would sound like an excuse.
Can -아서 take past tense?
No: 비가 와서 늦었어요, never 왔어서.
How do you say “but”?
Stem + -지만: 비싸지만 맛있어요. Sentence-initial: 하지만.
- National Institute of Korean Language, Standard Korean Language Dictionary (표준국어대사전) — entries for the connective endings -고, -아서/-어서, -(으)니까 and -지만.
- National Institute for International Education, TOPIK official site — TOPIK I, where these connectors are tested.
- National Institute of Korean Language, Revised Romanization of Korean.