아/어서 vs 니까: The Two Ways to Say Because in Korean

For Korean learners at beginner to intermediate level

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Korean has two main ways to say "because" or "so": 아/어서 and (으)니까. Most textbooks introduce both without explaining when one sounds wrong — and in Korean, using the wrong one isn't just unnatural, it's a grammatical error.

Here's the rule that actually matters.

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The Core Difference

아/어서 = a reason that flows naturally into a result (으)니까 = a reason you're giving to justify a command, suggestion, or opinion

That one distinction covers 90% of the cases you'll encounter.

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The Practical Test

Ask yourself: does what follows involve a command, a suggestion, or "let's"?

If yes → use (으)니까 If no → either works, but 아/어서 usually sounds more natural

✅ 추우니까 창문 닫아요. — It's cold, so close the window. ❌ 추워서 창문 닫아요. — Grammatically wrong before a request or command.

✅ 배가 고파서 밥을 먹었어요. — I was hungry, so I ate. ✅ 배가 고프니까 밥 먹자. — Since we're hungry, let's eat.

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Two More Rules Worth Knowing

1. Past tense + 서 = wrong

You can't attach -았/었 directly before 서.

❌ 어제 늦게 잠들었어서 피곤해요. ✅ 어제 늦게 잠들어서 피곤해요. (use the present form + 서, even when the meaning is past)

(으)니까 has no such restriction — 먹었으니까, 갔으니까 are both fine.

2. Tone: natural flow vs. explicit explanation

아/어서 feels like one thing leading into another. (으)니까 feels like you're actively giving a reason — slightly more deliberate, sometimes slightly defensive.

길이 막혀서 늦었어요. — Traffic was bad so I'm late. (natural, matter-of-fact) 길이 막혔으니까 늦은 거예요. — The reason I'm late is that traffic was bad. (explaining yourself)

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Quick Reference

	아/어서	(으)니까
Before commands / suggestions	❌	✅
After past tense (-았/었)	❌	✅
Feels like	natural flow	giving a reason

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Write a Korean sentence using 아/어서 or (으)니까 today — and see if the correction matches what you expected. Try Korean Diary AI →