Reasons: ~(으)니까, ~아/어서
Express reasons and causes with two different patterns and their nuances.
Korean has two main ways to express 'because' / 'so', and the choice carries strong nuance. -(으)니까 emphasises the speaker's reasoning or judgement — used when you're explaining why you decided something or justifying an action: 비가 오니까 우산을 가져가세요 (since it's raining, take an umbrella). -아/어서 expresses a more objective cause-effect chain — used when the reason is just a fact: 비가 와서 우산을 가져왔어요 (because it rained, I brought an umbrella).
The critical rule: -아/어서 cannot be used with command or suggestion endings, while -(으)니까 can. 'Because it's raining, let's go home' must use -(으)니까: 비가 오니까 집에 가요. The opposite is sometimes flagged on TOPIK exams. Most learners default to -아/어서, which works for explanations but sounds wrong for instructions. Build the habit of using -(으)니까 when giving advice or suggestions — it's the more natural choice in those contexts.
~아/어서 — Because (natural cause/reason)
Verb/adjective stem + 아/어서 gives a reason. Cannot be used with imperative (주세요) or propositive (갈까요?) endings. Used for objective/natural reasons: 배고파서 밥 먹었어요 (I ate because I was hungry).
~(으)니까 — Because / Since (subjective reason + commands OK)
Verb/adjective stem + (으)니까 also gives a reason, but CAN be used before commands and suggestions. Often more subjective/emphatic: 추우니까 문 닫아 주세요 (It's cold, so please close the door).
How It Works
Korean has two main ways to give a reason, and the difference is not stylistic — one of them is grammatically blocked in certain sentence types.
-아/어서 states a cause and follows the vowel rule you know: 배고파서, 바빠서, 좋아서. It presents the reason as objective and naturally consequential, and it is the default for explaining why something happened. Crucially, it cannot appear before an imperative or a propositive. 추워서 문 닫아 주세요 is ungrammatical.
-(으)니까 also gives a reason but can precede commands and suggestions: 추우니까 문 닫아 주세요 works. It frames the reason as the speaker's own judgement or as new information the listener should act on, which is why it pairs naturally with telling someone to do something.
That blocking rule is the single most useful thing to take from this lesson, because it is absolute. If your sentence ends in 주세요, -(으)세요, -(으)ㅂ시다 or -(으)ㄹ까요?, the reason clause must use -(으)니까.
There is also a politeness dimension. -(으)니까 can sound assertive when used to justify yourself to someone senior, so for apologies and explanations upward, -아/어서 is generally safer: 늦어서 죄송합니다 rather than 늦으니까.
Where You'll Hear It
Apologies and excuses are where this distinction gets tested daily. 늦어서 죄송합니다 ('sorry for being late') is the standard formula and uses -아/어서 precisely because it is an explanation offered humbly. Substituting -(으)니까 would sound like you were arguing your case.
-(으)니까 dominates when you are directing someone. 비 오니까 우산 가져가세요 ('it's raining, so take an umbrella') and 늦었으니까 택시 타자 ('we're late, so let's take a taxi') both need it, and both are the kind of practical instruction you give constantly.
In service situations you will hear -(으)니까 used to explain policy: 규정이니까 안 됩니다. It carries a note of 'this is simply how it is', which is why it can feel blunt when directed at you.
-아/어서 also appears in a non-causal use that confuses learners at first: it can simply mark sequence with a connection, as in 앉아서 기다리세요 ('sit and wait'). Here it is not giving a reason at all. Recognising this second use prevents you from hunting for a cause that is not there.
Cultural & Language Note
The 아/어서 versus 으니까 split is one of the places where Korean grammar encodes politeness. 늦어서 죄송합니다 is the standard apology; using 늦으니까 죄송합니다 sounds like you are justifying yourself rather than apologising. Because 으니까 foregrounds the speaker's reasoning, it can read as defensive in contexts where deference is expected.
Common Mistakes to Avoid
아/어서 cannot precede commands or suggestions. 비가 와서 집에 가세요 is ungrammatical; it must be 비가 오니까 집에 가세요. This is the single most tested point of the pair on proficiency exams. Learners default to 아/어서 because it appears first in most textbooks, then carry it into imperative sentences where only 으니까 works.
How to Practice This Lesson
Learn the blocking rule first, because it is the only part of this lesson that is absolutely determined. Write five sentences ending in 주세요 or -(으)세요 and attach a reason to each. Every single one must use -(으)니까. Doing this once mechanically is worth more than reading the explanation three times.
Then drill the apology formula as a fixed unit: 늦어서 죄송합니다, 기다리게 해서 죄송합니다. These are high-frequency and always take -아/어서, so having them memorised removes the decision entirely in the situations where you are most flustered.
For everything else, accept that the nuance takes time. When neither a command nor an apology is involved, both forms are often acceptable, and choosing between them is a matter of feel that develops through exposure rather than analysis. Spend your effort on the two clear cases and let the middle ground settle later.