Hangul 114: Giving Reasons with -아서/어서 and -(으)니까

내가 왜 그렇게 느끼는지, 왜 그렇게 했는지 이야기할 수 있어야 대화가 깊어진다. 지난 시간에 마음을 표현하는 법을 배웠다면, 오늘은 그 이유를 말해 보기로 하자.

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Before We Start

In Hangul 113, you learned how to say how you feel. The natural next step is saying why. In English, “because” comes before the reason: “I stayed home because it rained.” In Korean, the reason comes first, and a small ending attaches to it: 비가 와서 집에 있었어요.

This post shows you how to say “because” in Korean with its two most useful endings, -아/어서 and -(으)니까, and when to choose each one.

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Giving a Reason with -아/어서

-아/어서 is the everyday “because” and “so.” Add it to a verb or adjective stem. It follows the same vowel pattern as -아요/어요: use -아서 after ㅏ or ㅗ, -어서 after other vowels, and 해서 for 하다 words.

Dictionary form	With -아/어서	Meaning
좋다	좋아서	because it’s good
먹다	먹어서	because I ate
바쁘다	바빠서	because I’m busy
고프다	고파서	because I’m hungry
피곤하다	피곤해서	because I’m tired
걱정되다	걱정돼서	because I’m worried

Korean	English
피곤해서 일찍 잤어요.	I was tired, so I went to bed early.
비가 와서 집에 있었어요.	It rained, so I stayed home.
배가 고파서 라면을 먹었어요.	I was hungry, so I ate ramyeon.
서운해서 말을 못 했어요.	I felt hurt, so I couldn’t say anything.
주말이라서 사람이 많아요.	It’s the weekend, so there are lots of people.

With nouns, attach -(이)라서: 주말이라서 (because it’s the weekend), 학생이라서 (because I’m a student).

Two rules to remember. First, never put the past tense inside the reason. You say 비가 와서, not 비가 왔어서. The tense goes only at the end of the whole sentence. Second, -아/어서 cannot be followed by a command or a suggestion (-세요, -(으)ㄹ까요, -아/어요 as “let’s”). That job belongs to -(으)니까.

-아/어서 is also how you answer 왜? from Hangul 112: 왜 늦었어요? 차가 막혀서요. (Why were you late? Because of traffic.) It is also used in thanks and apologies: 도와줘서 고마워요. (Thanks for helping.) 늦어서 미안해요. (Sorry I’m late.) And you have already seen it linking actions in order, as in 친구를 만나서 영화를 봤어요.

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Giving a Reason with -(으)니까

-(으)니까 is the “because” you use when the reason supports what you are about to say or ask. Use -니까 after a vowel stem and -으니까 after a consonant stem.

Korean	English
추우니까 따뜻하게 입으세요.	It’s cold, so dress warmly.
시간이 없으니까 빨리 가요.	We have no time, so let’s hurry.
늦었으니까 택시 타요.	We’re late, so let’s take a taxi.
날씨가 좋으니까 산책할까요?	The weather is nice, so shall we take a walk?

Unlike -아/어서, -(으)니까 can follow a past tense (늦었으니까) and can be followed by commands and suggestions. If you are asking, suggesting, or telling someone what to do, reach for -(으)니까.

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Which One Should I Use?

	-아/어서	-(으)니까
Feels like	a natural cause and result	a reason behind what comes next
Followed by a command or suggestion	✗	✓
Past tense inside the reason	✗	✓
Thanks and apologies	✓	✗
Answering 왜?	✓	possible, but less common

A quick rule of thumb: describing what happened or how you feel? Use -아/어서. Asking someone to do something or proposing a plan? Use -(으)니까.

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Putting It Together

재민: 이번 주말에 코미디 영화 볼까요? (Shall we watch a comedy this weekend?)

은지: 토요일은 약속이 있어서 안 돼요. 일요일은 어때요? (I have plans on Saturday, so I can’t. How about Sunday?)

재민: 좋아요. 일요일에는 사람이 많으니까 일찍 가요. (Good. There will be lots of people on Sunday, so let’s go early.)

은지: 그런데 저는 일찍 못 일어나서 걱정돼요. (But I can’t wake up early, so I’m worried.)

재민: 그럼 제가 전화할까요? (Then shall I call you?)

은지: 네, 좋아요. 도와줘서 고마워요! (Yes, please. Thanks for helping!)

See how 약속이 있어서 simply explains, how 많으니까 leads straight into the suggestion 가요, and how 도와줘서 turns a reason into a thank-you. Each ending is doing a slightly different job.

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For Heritage Learners

You have probably heard -(으)니까 followed by an instruction more times than you can count, in the “it’s cold, so put on a jacket” kind of sentence. That is no accident. It is exactly where the ending belongs. The spelling is where heritage learners most often slip: write 돼서, not 되서 (it comes from 되어서), and 해서, not 했서. Remember too that the past tense never goes inside -아/어서.

For Beginners

Start with just -아/어서. Answer 왜? questions with it, and add 그래서 (so, and so) at the start of a sentence when you want to connect two ideas: 비가 와요. 그래서 집에 있어요. Come back to -(으)니까 once you want to invite or ask someone to do something, since that is where it matters most.

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Practice

1. Answer these two questions using -아/어서요: 왜 늦었어요? 왜 피곤해요?
2. Join each pair into one sentence with -아/어서: 비가 와요. 집에 있어요. / 배가 고파요. 밥을 먹어요.
3. Write a suggestion that starts with a reason using -(으)니까, for example about the weather or the time.
4. Thank someone for something using -아/어서 고마워요.
5. Pick a feeling word from Hangul 113 and say why you feel that way.

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What’s Next

Now that you can say why, Hangul 115 will move into making polite requests with -아/어 주세요, which pair naturally with the reasons you just learned.

오늘도 천천히, 한 걸음씩 함께 가요!