Side by side
아/어서 vs (으)니까: Two Ways to Say 'Because'
They translate the same. Only one can come before a command, and only one belongs in an apology.
Korean has two everyday ways to give a reason, and textbooks usually translate both as 'because'. That translation hides three rules that decide which one is correct.
One of them takes tense and the other refuses it. One can be followed by a command and the other cannot. And in an apology, only one of them will not make you sound like you are making excuses.
In one line
아/어서 = objective cause, never takes tense, never before commands. (으)니까 = your own judgement, takes tense, required before commands.
| Particle | Core meaning | Used with | Example |
|---|---|---|---|
| 아/어서 | Cause, or connected sequence | Statements only; stem never takes tense | 늦어서 죄송합니다 neujeoseo joesonghamnida I'm sorry for being late |
| (으)니까 | Reason as speaker's judgement | Statements, commands, suggestions; takes tense | 위험하니까 조심하세요 wiheomhanikka josimhaseyo It's dangerous, so be careful |
When to Use 아/어서
1. A Plain, Objective Cause
아/어서 presents the reason as a simple fact of the world rather than your opinion about it. This neutrality is why it is the default choice in writing and in polite speech.
비가 와서 못 갔어요.
biga waseo mot gasseoyo.
I couldn't go because it rained.
2. Apologies and Thanks
Every standard Korean apology and thank-you uses 아/어서. Swapping in 니까 turns an apology into a justification, which sounds defensive.
도와주셔서 감사합니다.
dowajusyeoseo gamsahamnida.
Thank you for helping me.
3. Connected Sequence: 'And Then'
아/어서 has a second job that has nothing to do with cause. It links two actions where the second happens at, or by means of, the result of the first. This is why 가서, 와서, 만나서 and 앉아서 are so common.
친구를 만나서 영화를 봤어요.
chingureul mannaseo yeonghwareul bwasseoyo.
I met my friend and (then, with them) watched a movie.
When to Use (으)니까
(으)니까 frames the reason as something you have concluded, which gives it two abilities 아/어서 lacks. It can carry tense (갔으니까, 먹었으니까), and it is the only one of the two that may be followed by a command, a suggestion, or an invitation. It also marks discovery: noticing something upon doing something else.
배고프니까 밥 먹으러 가요.
baegopeunikka bap meogeureo gayo.
I'm hungry, so let's go eat.
제가 벌써 했으니까 걱정하지 마세요.
jega beolsseo haesseunikka geokjeonghaji maseyo.
I already did it, so don't worry.
집에 가니까 아무도 없었어요.
jibe ganikka amudo eopseosseoyo.
When I got home, [I found] nobody was there.
The core difference
Objective Cause
“Stating a fact”
Speaker's Judgement
“Making a case for action”
Common mistakes
배고파서 밥 먹자
baegopaseo bap meokja
배고프니까 밥 먹자
baegopeunikka bap meokja
밥 먹자 is a suggestion. 아/어서 can never be followed by a command, request, or suggestion: only (으)니까 can.
늦었어서 죄송합니다
neujeosseoseo joesonghamnida
늦어서 죄송합니다
neujeoseo joesonghamnida
아/어서 never carries tense. The past tense belongs to the final verb, so the stem stays plain: 늦어서.
늦으니까 죄송합니다
neujeunikka joesonghamnida
늦어서 죄송합니다
neujeoseo joesonghamnida
In an apology, 니까 sounds like you are justifying yourself rather than apologising. Apologies and thanks always take 아/어서.
시간이 없어서 빨리 가세요
sigani eopseoseo ppalli gaseyo
시간이 없으니까 빨리 가세요
sigani eopseunikka ppalli gaseyo
가세요 is a command. Again, only (으)니까 may precede it.
비가 와서 우산을 가져가세요 / 비가 오니까 우산을 가져가세요
biga waseo usaneul gajyeogaseyo / biga onikka usaneul gajyeogaseyo
Both are heard, but 니까 is correct
Native speakers do occasionally break this rule in casual speech, especially with 와서. In an exam or in writing, always use 니까 before a command.
How to Choose in 5 Seconds
Ask these in order and stop at the first yes.
Frequently asked questions
Why can't 아/어서 take past tense?
Because it binds the two clauses into a single connected event, and the event as a whole is dated by the final verb. 비가 와서 못 갔어요 is already fully past: adding 왔어서 would date the same event twice.
Does (으)니까 sound rude?
It can, in the wrong place. Because it frames the reason as your own judgement, using it toward someone senior (especially in an apology) reads as talking back. In neutral contexts it is perfectly polite.
What is 그래서 and 그러니까?
They are the sentence-starting versions of the same two forms, and they follow the same rules. 그래서 continues a statement; 그러니까 leads into advice or a request, and in speech it also works as 'in other words'.
How do I attach (으)니까 correctly?
After a vowel or ㄹ use 니까 (가니까, 사니까 from 살다). After any other consonant use 으니까 (먹으니까). Past tense is always 았/었으니까.
Look them up in the dictionary
Both particles in the Basic Korean Dictionary from the National Institute of Korean Language, with English.