Korean -아서/어서: How to Say "Because" and "And Then"
Learn -아서/어서, the everyday Korean ending for "because" and "and then." See how to build it, why 았어서 is wrong, and when to use -(으)니까 instead.
Beginner8 min readTap any Korean word to see it explained
The short answer
Add -아서/어서 to a verb to mean “because” or “and then.” 바빠서 못 가요 means “I’m busy, so I can’t go.” Two rules: no past tense before it, and no commands or suggestions after it.
English uses separate words for “because” and “so.” Korean often just changes the end of the first verb and keeps going. You will hear -아서/어서 in almost every conversation, from apologies to excuses to stories about your weekend. Tap the endings below to see how it works.
1. How to make it
It follows the same vowel rule as the polite present tense. If you know 가요 and 먹어요, just swap 요 for 서.
- Last vowel ㅏ or ㅗ: add 아서
- Any other vowel: add 어서
- 하다 verbs: always 해서
| Verb | Meaning | Polite present | Because / and then |
|---|---|---|---|
| 가다 | to go | 가요 | 가서 |
| 오다 | to come | 와요 | 와서 |
| 많다 | to be many, a lot | 많아요 | 많아서 |
| 먹다 | to eat | 먹어요 | 먹어서 |
| 마시다 | to drink | 마셔요 | 마셔서 |
| 바쁘다 | to be busy | 바빠요 | 바빠서 |
| 춥다 | to be cold | 추워요 | 추워서 |
| 공부하다 | to study | 공부해요 | 공부해서 |
With a noun, use 이라서 after a consonant and 라서 after a vowel: 학생이라서 (because I’m a student), 친구라서 (because he’s a friend).
2. “Because”: giving a reason
Put the reason first and the result second. The English word “so” is built into the ending.
The past tense is only on the last verb. 배고파서 takes its time from 먹었어요.
3. Sorry and thank you
Koreans use -아서/어서 every day to say what they are sorry or thankful for. Learn these as set phrases.
For more respect, say 늦어서 죄송해요.
You will hear this at the start of every event, show and speech.
4. “And then”: one action leads to the next
The same ending also links two actions in order, when the first one sets up the second. Usually you go somewhere and then do something there, or you make something and then use it.
With 서, you drank the coffee with that friend. With 만나고, the two things could be unrelated.
5. The two rules learners break
Rule 1: no past tense before it. Even when the reason happened yesterday, the first verb stays plain. Korean puts the tense only at the end of the sentence.
| Wrong | Right | Meaning |
|---|---|---|
| 어제 아팠어서 학교에 못 갔어요 | 어제 아파서 학교에 못 갔어요 | I was sick yesterday, so I couldn’t go to school. |
| 비가 와서 우산을 가져가세요 | 비가 오니까 우산을 가져가세요 | It’s raining, so take an umbrella. |
| 피곤해서 집에 갈까요? | 피곤하니까 집에 갈까요? | We’re tired, so shall we go home? |
Rule 2: no commands or suggestions after the “because.” If the second half says “please do,” “let’s” or “shall we,” switch to -(으)니까. It also gives a reason, but it points at the listener a bit more, like “since, you know...”
6. 그래서 at the start of a sentence
그래서 is simply 그렇다 (to be so) plus -아서/어서. Use it to begin a new sentence with “so” or “that’s why.”
Keep going
Song lyrics are full of reasons: 보고 싶어서 (because I miss you), 사랑해서 (because I love you). Open a song in Hanbok Lyrics and tap through the lines to spot them. To link ideas with “but” instead, read Korean -지만 (but), and for the vowel rule behind 아서/어서, see the present tense.
Check yourself
Quick check · 1 of 4Which one is correct?
It rained, so I bought an umbrella.
Questions learners ask
How do you say "because" in Korean?
The most common way is to add -아서/어서 to the verb: 바빠서 (because I am busy), 비가 와서 (because it is raining). At the start of a sentence, use 그래서 ("so").
What is the difference between 아서/어서 and 니까?
Both give a reason. -아서/어서 is the neutral, everyday choice, but it cannot be followed by a command or a suggestion. -(으)니까 can: 추우니까 코트 입으세요 (It is cold, so wear a coat).
Can I say 았어서 for the past tense?
No. -아서/어서 never takes the past marker. The tense goes on the last verb only: 어제 아파서 학교에 못 갔어요 (I was sick yesterday, so I could not go to school).
When do I use 아서 and when 어서?
Use 아서 when the last vowel of the stem is ㅏ or ㅗ (가서, 와서, 많아서). Use 어서 for every other vowel (먹어서, 마셔서). For 하다 verbs, it is always 해서.