Both -아서/어서 and -(으)니까 mean "because", and in most sentences that simply report what happened either one is understood. The one test that decides between them in real conversation is what comes after the reason: if the second half suggests, asks or tells someone to do something, -아서/어서 does not work and you need -(으)니까.
That single test covers most of the mistakes we hear. The rest of this guide shows the test in use, the two places where -아서/어서 is still the only right choice, and the small set of form rules for each ending.
The one test: what comes next?
Say the reason, then look at the second half of your sentence.
If the second half is a plain statement of what happened or how things are, -아서/어서 is the natural choice. 버스가 안 와서 늦었어요: the bus did not come, so I was late. Nobody is being asked to do anything.
If the second half asks someone to do something, with -(으)세요, -(으)ㄹ까요?, -(으)ㅂ시다 or any other ending that suggests, requests or orders, the reason has to take -(으)니까. 비가 오니까 우산을 가져가세요: it is raining, so take an umbrella. The same sentence with 와서 is wrong, and a Korean listener hears it as wrong, not just as a little unusual.
- 버스가 안 와서 늦었어요.The bus didn't come, so I was late.1B
- 비가 오니까 우산을 가져가세요.It's raining, so take an umbrella.2A
- 시간이 없으니까 택시를 탑시다.We have no time, so let's take a taxi.2A
- 머리가 아파서 약을 먹었어요.My head hurt, so I took some medicine.1B
- 머리가 아프니까 집에서 쉬세요.Your head hurts, so rest at home.2A
Notice that the reason can be the same in both columns. 머리가 아파서 reports; 머리가 아프니까 prepares the listener for advice. The reason did not change. What follows it did.
- 버스가 안 와서 늦었어요.The bus didn't come, so I was late.
- 버스가 안 오니까 택시를 탈까요?The bus isn't coming, so shall we take a taxi?
- 늦어서 죄송합니다.I'm sorry I'm late.
- 주말이니까 같이 영화 볼까요?It's the weekend, so shall we see a film together?
Where -아서/어서 is the only choice
Two situations keep -아서/어서 even though the second half is not a plain report.
Thanks and apologies. 늦어서 죄송합니다, 만나서 반갑습니다. These are fixed, and -(으)니까 does not go in them. There is a reason it sounds wrong: -(으)니까 presents the reason as something you have weighed up and now expect the listener to accept. In an apology that comes across as justifying yourself rather than apologising. Keep -아서/어서 whenever you are thanking someone or saying sorry.
Answering 왜? on its own. When someone asks why and you give only the reason, you stop after -아서/어서 and add 요: 왜 늦었어요? 아침에 못 일어나서요. This short answer is one of the most useful sentence shapes in daily life, and it always uses -아서/어서.
- 늦어서 죄송합니다.I'm sorry I'm late.1B
- 만나서 반갑습니다.Nice to meet you.1A
- 왜 늦었어요? — 버스가 안 와서요.Why were you late? — Because the bus didn't come.1B
The forms, side by side
| -아서/어서 | -(으)니까 | |
|---|---|---|
| How to make it | take the -아요/어요 form, change 요 to 서: 와요 → 와서, 먹어요 → 먹어서, 공부해요 → 공부해서 | 받침 → 으니까 (많으니까); no 받침 → 니까 (비싸니까); ㄹ drops (멀다 → 머니까); ㅂ → 우 (춥다 → 추우니까) |
| After a noun | 이라서 / 라서: 시험이라서, 친구라서 | 이니까 / 니까: 주말이니까, 휴가니까 |
| What can follow | a statement of what happened or how things are | anything, including -(으)세요, -(으)ㄹ까요?, -(으)ㅂ시다 |
| Past tense in the reason | never: 와서 늦었어요, not 왔어서 | allowed: 어제 많이 걸었으니까 오늘은 쉬세요 |
| Thanks and apologies | yes: 늦어서 죄송합니다 | no |
| Short answer to 왜? | yes: 못 일어나서요 | not usual |
The past-tense row trips up more people than the ㄹ and ㅂ changes. With -아서/어서 the last verb carries the time, so the reason stays in its plain form even when everything happened yesterday: 어제 버스가 안 와서 늦었어요, never 안 왔어서. With -(으)니까 you may put -았/었- in front when the reason is already finished: 어제 많이 걸었으니까 오늘은 쉬세요.
- 어제 많이 걸었으니까 오늘은 쉬세요.You walked a lot yesterday, so rest today.2A
- 시끄러워서 창문을 닫았어요.It was noisy, so I closed the window.1B
- 시끄러우니까 창문을 닫을까요?It's noisy, so shall I close the window?2A
- 내일 같이 산에 갈까요?Shall we go to the mountain together tomorrow?
- 내일은 비가 오니까 주말에 갈까요?It'll rain tomorrow, so shall we go at the weekend?
- 좋아요. 그런데 왜 내일 비가 와요?Sure. But why is it raining tomorrow?
- 뉴스에서 봐서요.Because I saw it on the news.
- The grammar page for -(으)니까 read on a phone: the forms after 받침, ㄹ and ㅂ, the key difference from -아서/어서, and the textbooks that teach it.
The mistake we hear most
In our classes the two endings arrive in a fixed order. -아서/어서 comes first, in Level 1B, and students use it for every reason for a few weeks. -(으)니까 comes in the same level a little later and is extended in Level 2A. The mistake we hear most is the first ending overstaying its welcome: 비가 와서 우산을 가져가세요, 시간이 없어서 택시를 탑시다. The reason is fine; the ending is the one the student learned first.
Then the opposite happens. Once -(으)니까 feels comfortable, some students switch everything to it, including their apologies, and 늦으니까 죄송합니다 appears. That one makes Korean listeners wince, because it sounds as if you are explaining why your lateness was reasonable.
We tried, early on, teaching the pair with labels, a "cause" ending and a "reason" ending. It did not help. Students could repeat the labels and still choose wrong in the next sentence, because the labels describe the reason, and the reason is the half of the sentence that does not decide anything. What worked was dropping the labels and asking one question every time a student pauses before a reason: what comes next? If the answer is "I'm going to ask them to do something", -(으)니까. If the answer is "I'm just saying what happened", or the sentence is a thank-you or an apology, -아서/어서. That question is the whole method, and it is why this guide is built around it.
Our own pages divide the work the same way. The grammar page for -(으)니까 explains the forms and the key difference and gives examples. It does not drill you. The practice set in lesson 5 of Level 2A does, and the one in lesson 4 of Level 1B covers -아서/어서. Both sets are short, so expect to come back to them rather than finish them once.
Questions people ask
Can I use -(으)니까 in a plain statement, like 버스가 안 오니까 늦었어요?
You can, and people do, but it adds a tone: the reason sounds like a judgment you are presenting, and the listener may hear a hint of "obviously". For simply reporting what happened, -아서/어서 is the neutral choice, and it is the one we ask students to use first.
What about 그래서 and 그러니까?
They are the same two endings as separate words, used to start a new sentence. 그래서 links a result to what you just said; 그러니까 links a suggestion or request. The grammar pages show how two sentences with 그래서 or 그러니까 become one with -아서/어서 or -(으)니까.
Is 때문에 a third "because"?
Yes, and it belongs to a different lesson: it gives the reason with a noun, 비 때문에, and is taught later in Level 2A. For now, master the two verb endings and the one test that separates them.