Grammar
Nikka vs Aseo/Eoseo (니까 vs 아서/어서): Two Ways to Say "Because"
니까 is the one you need when the second half tells somebody to do something. 아서/어서 cannot be followed by a command or a suggestion, so that single test covers most of the difference, and it is a hard grammatical rule rather than a matter of nuance.
Both translate as because, which is why English speakers reach for whichever one they learned first and then hit a wall at exactly one place.
The test
비가 오니까 우산 가져가세요.
bi-ga o-ni-kka u-san ga-jyeo-ga-se-yo.
It's raining, so take an umbrella.
비가 와서 안 갔어요.
bi-ga wa-seo an ga-sseo-yo.
It was raining, so I didn't go.
Swapping them in the first sentence produces something a Korean speaker will not say. Swapping them in the second is a shade of meaning rather than an error.
The difference when both are allowed
When the second half is an ordinary statement, either can appear and they lean slightly differently.
- 아서/어서 presents the cause as background, something both of you can see. It is the softer one and it is the safer default.
- 니까 presents the cause as your reason, something you are putting forward. It can sound like you are making a case, which is fine when you are and pointed when you are not.
This is why 니까 shows up in explanations and arguments, and why a bare 니까 answer to a complaint can land harder than you meant.
⚠️ This is about 아서/어서 meaning because. The same ending also means and then, and that one sits in front of commands all the time: 가서 사세요, 앉아서 기다리세요. Same shape, different job.
One place to be careful
Apologies and thanks take 아서/어서. 늦어서 죄송합니다 is the ordinary form. 늦으니까 죄송합니다 sounds like you are explaining rather than apologising, which is not the effect anybody wants in that moment.
What to do with this
- Default to 아서/어서 for statements. It is the safer of the two and it covers most of what you will say.
- Switch to 니까 the moment the second half becomes an instruction or a suggestion. That is the rule, not a preference.
- Keep 아서 for apologies and thanks, where it is fixed.
- Do not try to feel the nuance before the rule is automatic. The rule is doing most of the work.
The short version
Instruction or suggestion in the second half means 니까. Plain statement means either, with 아서/어서 as the softer default. Apologies and thanks take 아서/어서, and that one is worth memorising as a set.
Check your sentence building
The free level test looks at how you understand and build Korean sentences, connectors included. It takes a few minutes and needs no account.
Take the level test →