Lesson 2

Reasons: ~(으)니까, ~아/어서

Express reasons and causes with two different patterns and their nuances.

Korean has two main ways to express 'because' / 'so', and the choice carries strong nuance. -(으)니까 emphasises the speaker's reasoning or judgement — used when you're explaining why you decided something or justifying an action: 비가 오니까 우산을 가져가세요 (since it's raining, take an umbrella). -아/어서 expresses a more objective cause-effect chain — used when the reason is just a fact: 비가 와서 우산을 가져왔어요 (because it rained, I brought an umbrella).

The critical rule: -아/어서 cannot be used with command or suggestion endings, while -(으)니까 can. 'Because it's raining, let's go home' must use -(으)니까: 비가 오니까 집에 가요. The opposite is sometimes flagged on TOPIK exams. Most learners default to -아/어서, which works for explanations but sounds wrong for instructions. Build the habit of using -(으)니까 when giving advice or suggestions — it's the more natural choice in those contexts.

~아/어서 — Because (natural cause/reason)

Verb/adjective stem + 아/어서 gives a reason. Cannot be used with imperative (주세요) or propositive (갈까요?) endings. Used for objective/natural reasons: 배고파서 밥 먹었어요 (I ate because I was hungry).

피곤해서 일찍 잤어요.pigonhaeseo iljjik jasseoyoI slept early because I was tired.

맛있어서 다 먹었어요.masissoseo da meogeosseoyoI ate it all because it was delicious.

~(으)니까 — Because / Since (subjective reason + commands OK)

Verb/adjective stem + (으)니까 also gives a reason, but CAN be used before commands and suggestions. Often more subjective/emphatic: 추우니까 문 닫아 주세요 (It's cold, so please close the door).

시간이 없으니까 빨리 가세요.sigani eopseulnikka ppalli gaseyoThere's no time, so go quickly.

맛있으니까 한번 먹어 보세요.masisseunikka hanbeon meogeo boseyoIt's delicious, so try it.

때문에ttaemunebecause of

Examples비 때문에 못 나갔어요.bi ttaemune mot nagasseoyoI couldn't go out because of the rain.

늦다neutdato be late

Examples버스가 늦어서 지각했어요.beoseuga neujeoseo jigakhaesseoyoI was late to class because the bus was late.

바쁘다bappeudato be busy

Examples요즘 바빠서 연락을 못 했어요.yojeum bappaseo yeollageul mot haesseoyoI couldn't contact you because I've been busy.

춥다chupdato be cold (weather)

Examples밖이 추우니까 따뜻하게 입으세요.bakki chuunikka ttatteuthage ibeuseyoIt's cold outside, so dress warmly.

걱정하다geokjeonghadato worry

Examples괜찮으니까 걱정하지 마세요.gwaenchaneunikka geokjeonghaji maseyoIt's fine, so don't worry.

기쁘다gippeudato be happy / glad

Examples시험에 합격해서 기뻐요.siheome hapgyeokaeseo gippeoyoI'm happy because I passed the exam.

Cultural & Language Note

The 아/어서 versus 으니까 split is one of the places where Korean grammar encodes politeness. 늦어서 죄송합니다 is the standard apology; using 늦으니까 죄송합니다 sounds like you are justifying yourself rather than apologising. Because 으니까 foregrounds the speaker's reasoning, it can read as defensive in contexts where deference is expected.

Common Mistakes to Avoid

아/어서 cannot precede commands or suggestions. 비가 와서 집에 가세요 is ungrammatical; it must be 비가 오니까 집에 가세요. This is the single most tested point of the pair on proficiency exams. Learners default to 아/어서 because it appears first in most textbooks, then carry it into imperative sentences where only 으니까 works.