Korean -고 vs -아/어서: Two Ways to Say "And Then"

Here is a sentence from a learner's diary. It's close enough that most people would read straight past it:

아침에 보통 하는 일을 해서 옷을 갈아입고 나갔어요.

The writer meant "I did my usual morning things, got changed, and went out." What a Korean reader actually gets is "Because I did my usual morning things, I got changed and went out." The morning routine suddenly becomes the reason for getting dressed, which is not a thing anyone said.

One connector, wrong job. -아/어서 and -고 both land as "and" in English translation, and English gives you no reason to keep them apart. Korean does.

-고: two actions that happen to be next to each other

-고 is the neutral one. It lists. The two clauses are separate events that share a timeline and nothing else.

밥을 먹고 학교에 갔어요. I ate and went to school.

숙제를 하고 친구를 만났어요. I did my homework and met a friend.

Eating and going to school are unrelated. You could describe them in two sentences and lose nothing. -고 just saves you a full stop.

-아/어서: the second action is built on the first

-아/어서 says the two clauses are hooked together. Whatever the first clause produced — a place you're now standing in, an object now in your hand, a person now next to you — carries into the second.

시장에 가서 과일을 샀어요. I went to the market and bought fruit there.

과일을 사서 먹었어요. I bought fruit and ate it.

친구를 만나서 영화를 봤어요. I met a friend and watched a movie with them.

Look at the bolded words. None of them are in the Korean. They come for free with -아/어서, and that is exactly what -고 does not give you.

The test that decides it

Ask: does the first clause still matter during the second one?

Sentence	First clause still active?	Connector
시장에 ___ 과일을 샀어요	Yes — you're buying at the market	가서
시장에 ___ 집에 갔어요	No — the market is finished	가고
과일을 ___ 먹었어요	Yes — you're eating that fruit	사서
과일을 ___ 집에 갔어요	No — the fruit is just in your bag	사고

Same two verbs each time. The connector alone decides whether the sentences are one event or two.

This is why 앉아서 먹었어요 ("sat down and ate") is normal Korean and 앉고 먹었어요 is not. You eat while still seated. The sitting hasn't finished.

The other -아/어서: because

The same ending also marks cause, which is where the diary sentence at the top went wrong.

비가 와서 못 갔어요. I couldn't go because it rained.

늦어서 죄송합니다. I'm sorry for being late.

You don't have to choose between "and then" and "because" when you write — Korean readers pick it up from whether the two clauses have a plausible cause relationship. But it does mean that if you use -아/어서 between two unrelated actions, a reader will hunt for a causal link and be briefly confused when there isn't one. That's the whole failure in 하는 일을 해서 옷을 갈아입고.

(For "because" specifically, -아/어서 and -(으)니까 divide up differently again — that's its own post.)

The rule that stops the most errors

-아/어서 never takes -았/었-.

비가 왔어서 못 갔어요. ❌ 비가 와서 못 갔어요. ✅

The tense lives on the final verb of the sentence and nowhere else. 못 갔어요 is already past, so the whole sentence is past. Marking it twice isn't emphasis — it's ungrammatical.

This catches people constantly, because the English is past in both halves and there's nothing in your first language telling you to drop it.

-고 is the opposite: it can carry -았/었-, but only when you're listing two facts rather than sequencing two actions.

어제는 비가 왔고 오늘은 맑아요. Yesterday it rained, and today it's clear.

For plain "I did this, then that," leave it off: 밥을 먹고 나갔어요, not 먹었고 나갔어요.

One more limit

-아/어서 in its "because" job can't sit in front of a command or a suggestion.

비가 와서 우산을 가져가세요. ❌ 비가 오니까 우산을 가져가세요. ✅ It's raining, so take an umbrella.

If the second half tells someone to do something, you need -(으)니까.

Check yourself

Fill in the blank with -고 or -아/어서:

1. 도서관에 ______ (가다) 공부했어요.
2. 숙제를 ______ (하다) 친구를 만났어요.
3. 어제 눈이 많이 ______ (오다) 길이 미끄러웠어요.
4. 커피를 ______ (사다) 공원에서 마셨어요.

Answers: 가서 (you study at the library — connected) · 하고 (two separate events) · 와서 (cause — and note it is not 왔어서) · 사서 (you drink that coffee).

Why this belongs in your diary

A diary is mostly a chain of actions: I got up, I ate, I went out, I came home. That chain is where -고 and -아/어서 live, so you will write them more than almost any other grammar point — several times per entry, every entry.

That's also what makes the mistake expensive. A wrong particle is a small error in one spot. A wrong connector quietly rewrites the logic of your sentence, and because it stays grammatical, nobody flags it and you keep doing it.

Tonight, write five sentences about your day using only -고. Then go back and look at each one: is the first half still going on during the second half? Every time the answer is yes, switch it to -아/어서 and read it again. That single pass is the fastest way to make the distinction automatic.