Giving Reasons: -아서/어서 vs -(으)니까
Learn to give reasons with -아서/어서 and -(으)니까, attach them correctly to any verb or adjective, and know when only one of them works.
Mr. Gaon · Grammar, vocabulary & TOPIKWhat you will learn in this lesson
- Attach -아서/어서 and -(으)니까 to verbs, adjectives and nouns
- Handle irregular stems such as 듣다, 덥다, 모르다 and 살다
- Know why -아서/어서 cannot take the past tense or a command
- Choose the natural form for apologies, thanks and suggestions
Hello again! Today we learn how to say why. Korean has two very common ways to give a reason, and they look similar in English. This one is easy to mix up, so we will go step by step: rules first, then examples, then practice.
Two ways to say 'because'
In English you can say 'because', 'so' or 'since'. In Korean, a reason is usually attached to the end of the first clause as a connecting ending. The two most common ones are -아서/어서 and -(으)니까.
Both mean 'because' or 'so'. The first clause is the reason and the second clause is the result. For example, 배가 고파서 밥을 먹었어요 means 'I was hungry, so I ate.'
The difference is not about meaning so much as about grammar limits. -아서/어서 is a natural, neutral cause. -(으)니까 is more like a reason you present to the listener, and it is more flexible. Let’s look at how each one attaches first.
because / so (a natural cause and its result)
- If the last vowel of the stem is ㅏ or ㅗ, add -아서: 가다 → 가서, 오다 → 와서, 좋다 → 좋아서.
- With any other vowel, add -어서: 먹다 → 먹어서, 마시다 → 마셔서, 길다 → 길어서.
- 하다 becomes 해서: 공부하다 → 공부해서, 피곤하다 → 피곤해서.
- Nouns: N + 이어서 after a consonant, N + 여서 after a vowel (학생이어서, 친구여서). In speech you will also hear 이라서/라서 (학생이라서, 친구라서).
- The tense always goes on the second clause, never on -아서/어서 itself.
- 비가 와서 집에 있었어요. biga waseo jibe isseosseoyo.It rained, so I stayed home.
- 이 가방은 가벼워서 좋아요. i gabangeun gabyeowoseo joayo.This bag is light, so I like it.
- 오늘은 일요일이어서 은행이 문을 닫았어요. oneureun iryoirieoseo eunhaengi muneul dadasseoyo.Today is Sunday, so the bank is closed.
because / since (a reason presented to the listener)
- Stem ends in a vowel: add -니까 (가다 → 가니까, 바쁘다 → 바쁘니까).
- Stem ends in a consonant: add -으니까 (먹다 → 먹으니까, 많다 → 많으니까).
- Stem ends in ㄹ: drop the ㄹ and add -니까 (살다 → 사니까, 멀다 → 머니까).
- Past tense is allowed: 갔으니까, 먹었으니까, 끝났으니까.
- Nouns: N + 이니까 after a consonant, N + 니까 after a vowel (주말이니까, 친구니까).
- 지금은 바쁘니까 나중에 전화할게요. jigeumeun bappeunikka najunge jeonhwahalgeyo.I’m busy now, so I’ll call you later.
- 시간이 없으니까 택시를 탑시다. [시가니 업쓰니까 택씨를 탑씨다] sigani eopseunikka taeksireul tapsida.We don’t have time, so let’s take a taxi.
- 숙제를 다 했으니까 이제 놀아도 돼요. sukjereul da haesseunikka ije norado dwaeyo.You finished your homework, so you can play now.
Conjugation table (including irregulars)
| Dictionary form | Type | -아서/어서 | -(으)니까 |
|---|---|---|---|
| 가다 | regular, ㅏ vowel | 가서 | 가니까 |
| 먹다 | regular, consonant | 먹어서 | 먹으니까 |
| 피곤하다 | 하다 verb | 피곤해서 | 피곤하니까 |
| 살다 | ㄹ stem | 살아서 | 사니까 |
| 듣다 | ㄷ irregular | 들어서 | 들으니까 |
| 덥다 | ㅂ irregular | 더워서 | 더우니까 |
| 낫다 | ㅅ irregular | 나아서 | 나으니까 |
| 모르다 | 르 irregular | 몰라서 | 모르니까 |
| 바쁘다 | ㅡ drop | 바빠서 | 바쁘니까 |
| 그렇다 | ㅎ irregular | 그래서 | 그러니까 |
| 학생이다 | noun + 이다 | 학생이어서 | 학생이니까 |
Notice that 그래서 and 그러니까 are also used alone at the start of a sentence to mean 'so' or 'that’s why'.
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Now the important part. The two forms are not always interchangeable. There are three simple rules, and if you remember them, you will avoid most mistakes.
Rule 1: No past tense before -아서/어서
- 어제 늦게 자서 피곤해요. eoje neutge jaseo pigonhaeyo. I went to bed late yesterday, so I’m tired. Correct. The time is clear from 어제, and -아서 stays in its base form.
- 어제 늦게 잤어서 피곤해요. (✘) eoje neutge jasseoseo pigonhaeyo. (✘) Incorrect form Do not put -았/었- before -아서/어서. Use 자서.
- 어제 늦게 잤으니까 오늘은 일찍 자요. eoje neutge jasseunikka oneureun iljjik jayo. You went to bed late yesterday, so go to bed early today. Correct. -(으)니까 can take the past tense.
Rule 2: No commands or suggestions after -아서/어서
- 추워서 창문을 닫았어요. chuwoseo changmuneul dadasseoyo. It was cold, so I closed the window. Correct. The second clause is a statement.
- 추워서 창문을 닫으세요. (✘) chuwoseo changmuneul dadeuseyo. (✘) Unnatural A command (-(으)세요) or a suggestion (-(으)ㅂ시다, -(으)ㄹ까요?) cannot follow -아서/어서.
- 추우니까 창문을 닫으세요. chuunikka changmuneul dadeuseyo. It’s cold, so please close the window. Correct. Use -(으)니까 when you ask, tell or suggest.
- 날씨가 좋으니까 산책할까요? nalssiga joeunikka sanchaekalkkayo? The weather is nice, so shall we take a walk? Correct. A suggestion needs -(으)니까.
Rule 3: Thanks and apologies prefer -아서/어서
- 늦어서 죄송합니다. neujeoseo joesonghamnida. I’m sorry for being late. Natural. Fixed expressions of thanks and apology use -아서/어서.
- 늦으니까 죄송합니다. (✘) neujeunikka joesonghamnida. (✘) Unnatural With -(으)니까, it sounds as if you are blaming the situation or justifying yourself.
- 도와주셔서 감사합니다. dowajusyeoseo gamsahamnida. Thank you for helping me. Very common. 도와주시다 → 도와주셔서.
- 만나서 반가워요. mannaseo bangawoyo. Nice to meet you. Literally 'because I met you, I’m glad.' Another fixed -아서 expression.
Why does -(으)니까 feel stronger?
-(으)니까 often sounds like 'since, as you know' or 'because of this, you see'. It points the listener to a reason. That is why it fits with commands and suggestions: you give a reason, then ask the person to do something.
Because of this, using -(으)니까 too often with older people or in formal situations can sound a little pushy, especially in sentences like 바쁘니까 빨리 하세요. In polite situations, softer endings help: 시간이 조금 부족하니까 서둘러 주시겠어요?
One more use: -(으)니까 after a movement or action can mean 'when I did this, I found that…'. For example, 집에 가니까 아무도 없었어요 means 'When I got home, nobody was there.' This is a discovery, not a reason. You will see it often in stories.
More examples
- 음악을 크게 들어서 귀가 아파요. eumageul keuge deureoseo gwiga apayo. I listened to music loudly, so my ears hurt. 듣다 → 들어서 (ㄷ irregular)
- 길을 몰라서 지도를 봤어요. gireul mollaseo jidoreul bwasseoyo. I didn’t know the way, so I looked at the map. 모르다 → 몰라서 (르 irregular)
- 감기가 다 나아서 다시 운동을 시작했어요. gamgiga da naaseo dasi undongeul sijakaesseoyo. My cold is completely better, so I started exercising again. 낫다 → 나아서 (ㅅ irregular)
- 집이 회사에서 머니까 일찍 나가야 해요. jibi hoesaeseo meonikka iljjik nagaya haeyo. My house is far from the office, so I have to leave early. 멀다 → 머니까 (ㄹ drops)
- 이 영화는 너무 슬퍼서 많이 울었어요. i yeonghwaneun neomu seulpeoseo mani ureosseoyo. This movie was so sad that I cried a lot. 슬프다 → 슬퍼서 (ㅡ drops)
- 주말이니까 늦잠을 자도 괜찮아요. [주마리니까 늗짜믈 자도 괜차나요] jumarinikka neutjameul jado gwaenchanayo. It’s the weekend, so it’s okay to sleep in.
- 사람이 많아서 줄을 오래 섰어요. [사라미 마나서 주를 오래 서써요] sarami manaseo jureul orae seosseoyo. There were a lot of people, so I stood in line for a long time.
- 내일 시험이 있으니까 오늘은 일찍 들어갈게요. naeil siheomi isseunikka oneureun iljjik deureogalgeyo. I have an exam tomorrow, so I’ll go home early today.
- 김치가 매우니까 조금만 드세요. gimchiga maeunikka jogeumman deuseyo. The kimchi is spicy, so please have just a little. 맵다 → 매우니까 (ㅂ irregular)
- 집에 와서 보니까 지갑이 없었어요. jibe waseo bonikka jigabi eopseosseoyo. When I came home and looked, my wallet was gone. Here 와서 means 'came and then', and 보니까 is a discovery.
A small warning: -아서/어서 also has a second meaning, 'and then (in the same place)', as in 학교에 가서 공부해요. Context tells you which meaning it is. Don’t worry, you will get used to it quickly.
Dialogue: Running late
Check quiz
Q1. Choose the correct sentence.
Q2. Fill in the blank: 비가 ___ 우산을 가져가세요.
Q3. Which is the natural way to thank someone for inviting you?
Q4. What is the -(으)니까 form of 만들다?
Q5. Fill in the blank: 날씨가 ___ 에어컨을 켰어요. (덥다)
Q6. What does this sentence mean? 문을 여니까 고양이가 있었어요.
Great work! Remember the three rules: no past tense before -아서/어서, no commands or suggestions after it, and -아서/어서 for thanks and apologies. Try making one sentence about your own day with each form.
Key points
- -아서/어서 and -(으)니까 both mean 'because / so'.
- -아서/어서 never takes -았/었-; put the tense on the last verb.
- After a command or suggestion, use -(으)니까, not -아서/어서.
- Thanks and apologies use -아서/어서: 늦어서 죄송합니다, 도와주셔서 감사합니다.
- ㄹ stems drop ㄹ before -니까: 살다 → 사니까, 멀다 → 머니까.
- -(으)니까 can also show a discovery: 집에 가니까 아무도 없었어요.
Recommended official free resources
- Korean Basic Dictionary (11 languages) — National Institute of Korean Language
- Nuri-Sejong Hakdang (free e-textbooks, Sejong Korean) — King Sejong Institute Foundation
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