Saying "because" in Korean: -아서 vs -니까
한국어로 읽기 →Compare two common phrases: 「늦어서 죄송합니다」 and 「늦었으니까 죄송합니다」. The first is textbook natural Korean. The second sounds defensive, almost like arguing. Yet English dictionaries translate both endings simply as "because".
Learners struggle with Korean reason connectors not because the concepts differ, but because each grammar pattern imposes strict constraints on what type of clause can follow.
Rule 1: Use -(으)니까 with commands and suggestions
This single boundary resolves 80 percent of learner mistakes. Whenever the second half of your sentence is an imperative, suggestion, or request ending in 「-세요」, 「-(으)ㅂ시다」, or 「-(으)ㄹ까요?」, you cannot use 「-아/어서」.
비가 오니까 우산을 가져가세요. (Correct: It is raining, so take an umbrella.)
비가 와서 우산을 가져가세요. (Incorrect — native speakers never say this.)
Notice this on subway announcements and road signs: 「위험하니까 조심하세요」 (It is dangerous, so please be careful). In customer service and safety instructions, 「-(으)니까」 is standard.
Rule 2: Use -아/어서 for greetings, apologies, and thanks
Social formulas require 「-아/어서」. Using 「-(으)니까」 when apologising or thanking someone makes it sound as though you are presenting an intellectual argument rather than expressing sincere emotion.
Saying 「도와줬으니까 감사합니다」 sounds bizarrely transactional. Memorise these everyday courtesy phrases as single chunks.
Rule 3: Past tense marking differs
Never attach past tense markers 「-았/었-」 directly inside 「-아/어서」. The tense is marked only on the final verb at the end of the sentence.
바빠서 못 갔어요. (Correct: I was busy, so I could not go.)
바빴어서 못 갔어요. (Incorrect — double past tense is redundant.)
어제 바빴으니까 오늘은 쉴게요. (Correct — -(으)니까 accepts past tense freely.)
What about 때문에?
「때문에」 connects directly to nouns (비 때문에), or to verbs using 「-기 때문에」 (춥기 때문에). It provides a neutral, objective explanation and is especially common in formal writing and news broadcasts.