-아/어서 vs -(으)니까: Which Korean "Because" To Use
-아/어서 gives a neutral reason; -(으)니까 presses one. Only one works before commands and apologies. The rule, the traps, and practice at kdiaryai.com.
For Korean learners at beginner to lower-intermediate level
Both of these mean "because." Both are correct Korean. And for about eighty percent of the sentences you'll write in a diary, either one will pass.
That eighty percent is the problem. It teaches you that they're interchangeable, and then one day you write this:
❌ 늦었으니까 죄송합니다.
You meant "I'm sorry I'm late." What a Korean ear hears is closer to "I was late, so — sorry, I guess." The apology arrives with a shrug attached.
There are exactly three places where the choice is not free. Learn those three and you can keep guessing everywhere else.
The Feel, in One Line
-아/어서 reports a reason. -(으)니까 presses one.
-아/어서 sets a cause down on the table and lets the listener look at it. -(으)니까 hands it to them and expects them to act on it. That difference is why the three hard rules below fall out the way they do.
- 비가 와서 안 갔어요. — It rained, so I didn't go. (just telling you what happened)
- 비가 오니까 우산 가져가세요. — It's raining, so take an umbrella. (I'm telling you this so you'll do something)
Rule 1 — Commands and Suggestions Take -(으)니까 Only
If the second half of your sentence tells someone to do something (-(으)세요) or invites them to do it together (-(으)ㅂ시다, -(으)ㄹ까요?), -아/어서 is ungrammatical. Not unnatural — actually wrong.
| ❌ Wrong | ✅ Right |
|---|---|
| 바빠서 먼저 가세요 | 바쁘니까 먼저 가세요 |
| 시간이 없어서 택시를 탑시다 | 시간이 없으니까 택시를 탑시다 |
| 피곤해서 좀 쉴까요? | 피곤하니까 좀 쉴까요? |
This follows from the feel: a command is already pushing the listener. The reason attached to it has to push in the same direction.
Quick test: does your sentence end in -세요, -ㅂ시다, or -ㄹ까요? → -(으)니까.
Rule 2 — Apologies and Greetings Take -아/어서 Only
These are fixed. Korean speakers do not vary them.
- ✅ 만나서 반갑습니다. — Nice to meet you.
- ✅ 늦어서 죄송합니다. — Sorry I'm late.
- ✅ 도와주셔서 감사합니다. — Thank you for helping me.
- ✅ 걱정을 끼쳐서 미안해요. — Sorry for worrying you.
Swap in -(으)니까 and every one of them curdles:
❌ 만나니까 반갑습니다 · ❌ 늦었으니까 죄송합니다 · ❌ 도와주시니까 감사합니다
Why? Because you're not pressing a reason on the person — you're offering one, humbly. -(으)니까 in an apology reads as justifying yourself, which is the opposite of apologizing.
⚠️ This one bites hardest when you're writing to someone senior. A reason given with -(으)니까 to a boss or a teacher can land as an excuse even when the rest of the sentence is perfectly polite.
Rule 3 — Past Tense Attaches to -(으)니까, Never to -아/어서
This is pure mechanics, and it's the error I see most often in diary entries.
-아/어서 cannot carry -았/었-. The tense lives in the second clause only.
| ❌ Wrong | ✅ Right | English |
|---|---|---|
| 밥을 먹었어서 배불러요 | 밥을 먹어서 배불러요 | I ate, so I'm full |
| 어제 갔어서 오늘은 안 가요 | 어제 가서 오늘은 안 가요 | I went yesterday, so not today |
| 비가 왔어서 취소했어요 | 비가 와서 취소했어요 | It rained, so we cancelled |
-(으)니까 has no such restriction:
- ✅ 밥을 먹었으니까 배불러요.
- ✅ 어제 갔으니까 오늘은 안 가요.
So when you genuinely need to mark the first clause as past and finished, -(으)니까 is the one that lets you.
The Bonus Meaning: -(으)니까 as "and then I found out"
-(으)니까 has a second job that -아/어서 doesn't share at all. Attached to a verb of moving or looking, it means "when I did X, I discovered Y."
- 집에 가니까 아무도 없었어요. — When I got home, nobody was there.
- 창문을 여니까 눈이 오고 있었어요. — I opened the window and it was snowing.
- 먹어 보니까 생각보다 맛있었어요. — I tried it, and it was better than I expected.
There's no "because" in any of those. If you've been reading every -(으)니까 as a reason, this is why some sentences stopped making sense.
Note that this usage almost always takes no past marker on the first clause even though the event is past — 가니까, not 갔으니까. That's the tell.
And While We're Here: -아/어서 Also Means "and then"
Symmetrically, -아/어서 has a non-causal use — two actions where the first one enables the second.
- 카페에 가서 공부했어요. — I went to a cafe and studied (there).
- 앉아서 기다리세요. — Have a seat and wait.
- 친구를 만나서 밥을 먹었어요. — I met a friend and we ate.
⚠️ Notice 앉아서 기다리세요 ends in -세요 and still uses -아/어서. Rule 1 isn't violated, because this isn't a reason — it's a sequence. Rule 1 only governs -아/어서 when it means "because."
The difference: in the sequence use, the same subject does both actions, and the first one has to happen for the second to be possible. You can't wait-in-a-seated-way without sitting first.
A 20-Second Self-Check
Run this before you post your next entry:
- Does the sentence end in -세요 / -ㅂ시다 / -ㄹ까요? → -(으)니까
- Is it 죄송합니다 / 감사합니다 / 반갑습니다? → -아/어서
- Did I write -았어서 or -었어서? → delete the -았/었-, or switch to -(으)니까
- Am I saying "and then," not "because"? → -아/어서 for a sequence, -(으)니까 for a discovery
- None of the above? → either one works. Pick the one you'd say out loud.
Four checks and a shrug. That last item is not a cop-out — it's the actual state of the language.
Practice It Where It Counts
You can hold five rules in your head for about a day. What makes them stick is writing a real sentence about your real afternoon, getting it back with the particle circled, and seeing which of the five you skipped.
That's what kdiaryai.com is for. Write your diary entry, get it corrected in about five seconds, and watch the pattern in your own mistakes rather than in a textbook's.
Start with one sentence today:
오늘 아침에 지하철이 너무 복잡해서 조금 늦었어요.
Reason, not a command, no past marker on the first clause. You just used Rule 3 correctly without thinking about it.