Reason -아/어서 in Korean

Overview

The connective ending -아/어서 expresses reason or cause in Korean, equivalent to "because" or "so." This CEFR A2 pattern follows the same vowel harmony rules as the polite ending: -아서 after ㅏ/ㅗ stems, -어서 after other vowels, and 해서 for 하다 verbs. The first clause states the reason, and the second clause states the result.

An important restriction is that -아/어서 cannot be used with imperative or propositive (let's) endings in the result clause. For those situations, Korean uses -(으)니까 instead. This distinction is a common test point and a frequent source of errors.

-아/어서 also carries a nuance of natural or objective causation, as opposed to the more subjective or assertive -(으)니까. Both are essential for expressing reasons at the A2 level.

How It Works

Stem	+ 아/어서	Example
바쁘- (busy)	바빠서	바빠서 못 가요 (can't go because busy)
늦- (late)	늦어서	늦어서 죄송합니다 (sorry for being late)
맛있- (delicious)	맛있어서	맛있어서 다 먹었어요 (ate all because delicious)
하- (do)	해서	피곤해서 쉬었어요 (rested because tired)

Key restrictions:

• No tense marking on the -아/어서 clause
• Cannot use with imperative (do X!) or propositive (let's X!) endings
• Cannot use with past tense in the reason clause

Examples in Context

Korean	Romanization	English	Note
바빠서 못 가요.	ba-ppa-seo mot ga-yo	(I) can't go because (I'm) busy.	reason + result
늦어서 죄송합니다.	neu-jeo-seo joe-song-ham-ni-da	Sorry for being late.	apology
맛있어서 다 먹었어요.	ma-si-sseo-seo da meo-geo-sseo-yo	(I) ate all because it was delicious.	past result
피곤해서 집에 있을 거예요.	pi-gon-hae-seo ji-be i-sseul geo-ye-yo	(I'll) stay home because (I'm) tired.	future result
비가 와서 못 나갔어요.	bi-ga wa-seo mot na-ga-sseo-yo	Couldn't go out because it rained.	weather reason
만나서 반가워요.	man-na-seo ban-ga-wo-yo	Nice to meet you (happy to meet).	set phrase
시간이 없어서 빨리 먹었어요.	si-ga-ni eop-seo-seo ppal-li meo-geo-sseo-yo	(I) ate quickly because there was no time.	reason
좋아해서 자주 가요.	jo-a-hae-seo ja-ju ga-yo	(I) go often because (I) like it.	reason + habit
몰라서 물어봤어요.	mol-la-seo mu-reo-bwa-sseo-yo	(I) asked because (I) didn't know.	reason

Common Mistakes

Using -아/어서 with imperative endings

• Wrong: 비가 와서 우산 가져가세요.
• Right: 비가 오니까 우산 가져가세요. (Since it's raining, take an umbrella.)
• Why: -아/어서 cannot precede commands or suggestions. Use -(으)니까 instead.

Adding tense to the -아/어서 clause

• Wrong: 바빴어서 못 갔어요.
• Right: 바빠서 못 갔어요.
• Why: The -아/어서 clause does not carry tense; the tense of the whole sentence is determined by the final verb.

Confusing -아/어서 (reason) with -아/어서 (sequence)

• Wrong: Thinking 만나서 in 만나서 반가워요 means "because"
• Right: Here -아/어서 means "and then" (sequential): "meeting (you) and being glad"
• Why: -아/어서 has two uses: reason/cause and sequential action. Context determines which meaning applies.

Usage Notes

-아/어서 is the more common reason connector in casual conversation. It sounds natural and objective. The set phrase 만나서 반가워요 (nice to meet you) uses the sequential meaning. In contrast, -(으)니까 sounds more assertive and is often used when the speaker is justifying or persuading. Both are correct for stating reasons, but only -(으)니까 works with commands and suggestions.

Practice Tips

• Practice giving reasons for daily decisions: 배고파서 밥 먹었어요. 피곤해서 일찍 잤어요.
• Try converting -아/어서 sentences to -(으)니까 and notice which ones are valid (hint: those with commands are not valid with -아/어서).
• Memorize the set phrases: 만나서 반가워요, 늦어서 죄송합니다.

Related Concepts

• Prerequisite: Polite Ending -아/어요 — same vowel harmony rules apply
• Next steps: Reason -(으)니까 — the assertive reason pattern that works with commands