-아서 · -(으)니까 · -는데 · -지만 — which one?
Korean connectives compared: Connecting two clauses is where Korean sentences come to life. Beginners frequently struggle because English uses “because” or “but” for everything, while Korean has four distinct connectives with strict grammar rules regarding sentence endings, tense markers, and conversational etiquette.
1. 4-Way Comparison Table
| Connective | Core Meaning & Nuance | Commands / Suggestions (-(으)세요 / -(으)ㅂ시다) | Tense Marker (-았/었-) Allowed Before | Example Sentence |
|---|---|---|---|---|
| -아서 / -어서 | Natural cause & effect; sequential action | ✗ (for cause) | ✗ can’t | 배가 고파서 밥을 먹어요. |
| -(으)니까 | Speaker’s reasoning, justification, grounds | ✓ ok | ✓ ok | 비가 오니까 우산을 가져가세요. |
| -는데 / -(으)ㄴ데 | Background context, conversational premise, mild contrast | ✓ ok | ✓ ok | 저는 바쁜데 친구를 만나요. |
| -지만 | Direct factual contrast (“A, but B”) | ✓ ok | ✓ ok | 한국어가 어렵지만 재미있어요. |
-아서 / -어서
Natural cause & effect; sequential action
Commands / Suggestions (-(으)세요 / -(으)ㅂ시다)
✗ (for cause)
Tense marker before (-았/었-)
✗ can’t
배가 고파서 밥을 먹어요.
-(으)니까
Speaker’s reasoning, justification, grounds
Commands / Suggestions (-(으)세요 / -(으)ㅂ시다)
✓ ok
Tense marker before (-았/었-)
✓ ok
비가 오니까 우산을 가져가세요.
-는데 / -(으)ㄴ데
Background context, conversational premise, mild contrast
Commands / Suggestions (-(으)세요 / -(으)ㅂ시다)
✓ ok
Tense marker before (-았/었-)
✓ ok
저는 바쁜데 친구를 만나요.
-지만
Direct factual contrast (“A, but B”)
Commands / Suggestions (-(으)세요 / -(으)ㅂ시다)
✓ ok
Tense marker before (-았/었-)
✓ ok
한국어가 어렵지만 재미있어요.
2. Deep Dive & Common Traps
① -아서 / -어서 (Cause & Sequence / Means)
- Cause and sequential action: Expresses reasons (both subjective and objective, e.g. 배가 고파서 먹어요, 좋아서 샀어요) as well as sequence or means (가서 쉬세요, 사서 먹어요).
- No commands or suggestions when expressing cause: When expressing cause or reason, do not follow it with -(으)세요 or -(으)ㅂ시다 — use -(으)니까 instead (×비가 와서 우산을 가져가세요 → ○비가 오니까 우산을 가져가세요). Note that when -아서/어서 expresses sequence or means, commands are completely natural (집에 가서 푹 쉬세요).
- Past tense marker (-았/었-): In beginner Korean, causal -아서/어서 usually does not take the past tense pre-final ending (use 와서, not 왔어서); past tense is marked on the sentence’s final verb (○비가 와서 집에 있었어요). While modern spoken Korean sometimes uses -었어서 colloquially, standard learning avoids attaching past markers before -아서/어서.
- Social formulas & greetings: Always use -아서/어서 for greetings, thanks, and apologies: 만나서 반갑습니다 (Pleased to meet you), 늦어서 죄송합니다 (Sorry for being late).
② -(으)니까 (Reasoning, Justification & Commands)
- Speaker's reasoning and basis for action: Introduces the speaker’s grounds for a judgment, request, command, or proposal.
- The command-friendly connective: Works naturally with -(으)세요 (Please do...) and -(으)ㅂ시다 (Let’s...): 비가 오니까 우산을 가져가세요. (Because it is raining, please take an umbrella.)
- Freely combines with past tense: 밥을 먹었으니까 커피를 마실까요? (Since we ate, shall we drink coffee?)
- Do not use for polite apologies: Saying ×늦으니까 죄송합니다 sounds abrasive (like “Since I’m late, you get an apology”). Use 늦어서 죄송합니다.
③ -는데 / -(으)ㄴ데 (Background & Mild Contrast)
- Setting the stage: Tells the listener the background circumstance before asking a question, making an invitation, or explaining an event.
- Verb vs Adjective morphology:
- Verbs & words with 있다/없다 (including 맛있다, 재미있다): -는데 (먹는데, 가는데, 있는데, 맛있는데)
- Adjectives (descriptive verbs): -(으)ㄴ데 (바쁜데, 좋은데, 예쁜데)
- -는데 sets background context and leaves the conversational floor open, whereas -지만 establishes a direct, sharp contrast.
④ -지만 (Direct Opposition & Contrast)
- Factual contrast: Directly opposes two conflicting facts or qualities.
- 한국어가 어렵지만 재미있어요. (Korean is difficult, but it is fun.)
- Clear, neutral, and matter-of-fact.
3. 3-Second Quick Decision Flowchart
- Does the sentence end with a command (“Please do”, -(으)세요) or suggestion (“Shall we / Let’s”, -(으)ㄹ까요 / -(으)ㅂ시다)? → Use -(으)니까.
- Is it a social greeting or apology (“Sorry I am late”, “Nice to meet you”)? → Use -아서/어서.
- Are you setting background context before asking or telling (“I’m busy right now, but...”)? → Use -는데 / -(으)ㄴ데.
- Are you cleanly contrasting two opposite facts (“It is hard, but fun”)? → Use -지만.
Mini Quiz (6 Questions)
Choose the most natural connective for each context. Tap any option to check your answer and see the full explanation.