-아/어서: Because (Objective Cause)

-아/어서 is the first "because" most learners meet, and for good reason: it is the everyday way to state a reason in Korean. 비가 와서 안 갔어요 — "it rained, so I didn't go." But this ending has two hard rules baked into it that English has no parallel for, and both come from the same source: -아/어서 presents its reason as an objective, natural cause, like weather causing wet pavement. It is impersonal by nature. Once you feel that, the two rules stop being arbitrary restrictions and start being obvious.

Forming it: the same -아/어 you already know

The connective attaches to the verb stem using the standard -아/어 vowel harmony:

• Stem vowel ㅏ or ㅗ → -아서 (가다 → 가서, 좋다 → 좋아서)
• All other stem vowels → -어서 (먹다 → 먹어서, 있다 → 있어서)
• 하다 verbs → 해서 (피곤하다 → 피곤해서)

In each case the first clause is a plain fact of the world (rain fell, a stomach hurt, traffic backed up) and the second clause follows from it as a natural consequence. There is no sense of the speaker deciding anything — the cause is just there, and the effect just happens.

Rule 1: no tense marker on 서

The past (or future) tense lives only on the final verb of the sentence. You do not mark it on the -아/어서 clause, even when the whole event is clearly past. 비가 와서 … 갔어요, never ×비가 왔어서.

The whole sentence is about yesterday, yet 피곤해서 stays tenseless and only 잤어요 carries -았-. This feels wrong to English speakers, who mark tense on every verb ("I was tired, so I went..."). But in Korean the -아/어서 clause is not a full independent statement of a past fact — it is a reason feeding into the main event, and the main event's tense covers the whole thing. Keep 서 clean.

Rule 2: the second clause can't be a command or a suggestion

This is the rule that separates -아/어서 from its rival -(으)니까, and it is the source of a very common learner error. You cannot follow -아/어서 with an imperative (a command) or a propositive (a "let's" suggestion). These are ungrammatical:

• ×배고파서 밥 먹읍시다 ("I'm hungry, so let's eat")
• ×피곤해서 쉬세요 ("You look tired, so rest")

Why? Because -아/어서 frames its clause as an impersonal, factual cause — and an impersonal fact has no standing to tell someone what to do. A command or a suggestion is an act of the speaker's will, and it needs a reason rooted in the speaker's judgment. -아/어서 deliberately strips out that personal judgment, so it simply cannot license "so do this" or "so let's do this." To attach a reason to a command or suggestion, you switch to -(으)니까, which foregrounds exactly the speaker's reasoning that -아/어서 suppresses:

Ordinary statements, of course, are completely fine after -아/어서 — that is its home turf:

Where -아/어서 is the only option: apologies, thanks, greetings

Here is the flip side, and it is just as strict. A whole set of fixed social formulas — apologies, thanks, and "nice to meet you" — require -아/어서 and reject -(으)니까. Using 니까 here sounds like you are making excuses or justifying yourself, which is exactly the wrong tone for an apology or a thank-you.

Say ×늦으니까 죄송합니다 and a Korean listener hears something like "Since I'm late — I'm sorry," as if you were reasoning your way toward an apology rather than simply feeling sorry because you're late. The -아/어서 version is warm and direct; the 니까 version is cold and defensive. This is not a subtle stylistic preference — it is a fixed rule of politeness. Memorize these frames with 서 built in.

Why English speakers get this wrong

English "because" and "so" are utterly neutral about mood — "You're tired, so rest" and "You're tired, so you left early" use the same "so." Korean forces you to pick between an impersonal cause (-아/어서) and a reasoned justification (-(으)니까) before you even reach the second clause, and the choice is dictated by what that second clause is going to do. There is no shortcut around learning both endings and the division of labor between them. The good news: the rule is mechanical. Statement → 아/어서 is safe; command/suggestion → 니까 is required; apology/thanks → 아/어서 is required.

Causal vs. sequential -아/어서

The same string -아/어서 also builds a sequential "and then" (covered on -아/어서 for sequence). The two share a shape but not a meaning, and context tells them apart. Sequential -아/어서 links two actions where the second makes use of the result of the first — you go somewhere and then do something there, or make something and then use it:

That is not causal — buying did not cause eating; the buying produced the very thing that got eaten. Causal -아/어서, by contrast, gives a reason the second clause follows from: 아이스크림이 너무 달아서 못 먹었어요 ("the ice cream was too sweet, so I couldn't eat it"). A quick test: if the second clause reuses the object or place of the first, it is sequential; if it follows as an effect, it is causal. Conveniently, both readings obey the same two rules on this page — no tense on 서, and no command or suggestion after it.

Common Mistakes

1. Marking tense on 서. Tense belongs on the final verb only.

2. Following -아/어서 with a command. An imperative after 서 is ungrammatical; switch to 니까.

3. Following -아/어서 with a suggestion. Same problem with a "let's" clause.

4. Using 니까 in an apology or thank-you. The fixed social frames demand -아/어서.

5. Choosing the wrong harmony vowel. 좋다 has stem vowel ㅗ, so it takes -아서 (좋아서), while 먹다 takes -어서 (먹어서).

Key Takeaways

• -아/어서 states a reason as an objective, natural cause — the impersonal "because."
• No tense on 서 — the past/future sits on the final verb (와서 … 갔어요, never ×왔어서).
• The final clause cannot be a command or a suggestion; for those, switch to -(으)니까.
• But apologies, thanks, and greetings require -아/어서 (늦어서 죄송합니다, 만나서 반가워요) — 니까 there sounds like an excuse.
• The full head-to-head is on the -아/어서 vs. -(으)니까 page.