Lesson 60 · -아/어서 vs -(으)니까

HYEJU LANGUAGE LAB · BEGINNER · 60

Use -아/어서 vs -(으)니까 in the context of reasons, advice and suggestions. Work through the form, the meaning and the examples before trying the six practice questions.

Goal: choose the correct form for the situation, then make a natural sentence of your own.

How the pattern works

Reason / 理由

Both forms can connect a reason to a result. In the basic reason pattern, use -(으)니까 before a command or suggestion: 늦었으니까 빨리 가세요, not 늦어서 빨리 가세요.

-아/어서

Use 아서 after ㅏ/ㅗ, otherwise 어서; 하다 → 해서. This often presents a straightforward reason/result: 바빠서 못 갔어요. In the basic pattern, past time is normally shown at the end: 아파서 쉬었어요.

-(으)니까

Vowel + 니까: 가니까. Consonant + 으니까: 먹으니까. Final ㄹ drops: 살다 → 사니까. Past forms are possible: 갔으니까, 먹었으니까. A noun takes (이)니까: 학생이니까, 친구니까.

Reason vs sequence / 理由と順序

Do not confuse causal -아/어서 with a linked sequence. 가서 보세요 is natural: “Go and have a look.” The restriction taught here concerns the reason use, not every sentence containing 서.

Examples in context

늦었으니까 빨리 가세요.

neujeosseunikka ppalli gaseyo

You are late already, so please go quickly.

피곤해서 집에서 쉬었어요.

pigonhaeseo jibeseo swieosseoyo

I was tired, so I rested at home.

추우니까 따뜻하게 입으세요.

chuunikka ttatteuthage ibeuseyo

It is cold, so dress warmly.

바빠서 친구를 못 만났어요.

bappaseo chingureul mot mannasseoyo

I was busy, so I could not meet my friend.

비가 오니까 우산을 가져가세요.

biga onikka usaneul gajyeogaseyo

It is raining, so take an umbrella.

시간이 없어서 택시를 탔어요.

sigani eopseoseo taeksireul tasseoyo

I had no time, so I took a taxi.

여기는 조용하니까 여기에서 공부하자.

yeogineun joyonghanikka yeogieseo gongbuhaja

It is quiet here, so let’s study here.

학교가 머니까 일찍 출발하세요.

hakgyoga meonikka iljjik chulbalhaseyo

The school is far away, so leave early.

For a beginner contrast, first check the final clause: is it reporting a result, or asking/proposing an action? This is more useful than translating both endings as “because” and treating them as identical. Thanking someone commonly uses -아/어서: 와 줘서 고마워요.

Compare with similar grammar

LESSON 61

-기 때문에

교통사고가 났기 때문에 길이 막혀요.

A third option: more formal, puts weight on the cause, and also can’t be followed by a command.

Look ahead: Lesson 61 →

LESSON 63

-(으)ㄴ/는데

비가 오는데 우산 있어요?

Gives background before a question or request. Softer than giving a reason.

Look ahead: Lesson 63 →

LESSON 58

-아/어서 (sequence)

공원에 가서 산책했어요.

Remember: -아/어서 also links steps (go there, then walk). -(으)니까 never does this.

Review Lesson 58 →

LESSON 59

-(으)니까 (discovery)

문을 여니까 고양이가 있었어요.

With past situations, -(으)니까 can mean “when I…, I found that…”.

Review Lesson 59 →

Pattern	Command after it?	Past tense before it?	Apologies & thanks
-아/어서	No	No: 바빠서	Natural: 늦어서 죄송해요
-(으)니까	Yes	Yes: 바빴으니까	Sounds like an excuse
-기 때문에	No	Yes: 바빴기 때문에	Rare

Dialogue: Running Late for a Movie

Minsu arrives late to meet Yuna. Listen for -아/어서 when they explain what happened, and -(으)니까 when they suggest what to do next.

유나

민수 씨, 왜 늦었어요?minsu ssi, wae neujeosseoyo?Minsu, why were you late?

민수

미안해요. 길이 막혀서 늦었어요.mianhaeyo. giri makyeoseo neujeosseoyo.Sorry. The traffic was bad, so I was late.

유나

괜찮아요. 영화가 곧 시작하니까 빨리 들어가요.gwaenchanayo. yeonghwaga got sijakanikka ppalli deureogayo.That’s okay. The movie is about to start, so let’s go in quickly.

민수

네. 그런데 배가 고파서 팝콘을 사고 싶어요.ne. geureonde baega gopaseo papkoneul sago sipeoyo.Okay. But I’m hungry, so I want to buy popcorn.

유나

시간이 없으니까 영화 끝나고 같이 밥 먹어요.sigani eopseunikka yeonghwa kkeunnago gachi bap meogeoyo.We don’t have time, so let’s eat together after the movie.

민수

좋아요. 기다려 줘서 고마워요!joayo. gidaryeo jwoseo gomawoyo!Sounds good. Thanks for waiting for me!

Point: 막혀서 and 고파서 explain a result. 시작하니까 and 없으니까 come before a suggestion (“let’s …”). 기다려 줘서 고마워요 uses -아/어서 because thanks and apologies almost always take -아/어서.

Which One Should I Use? A Quick Guide

Look at the second half of the sentence first.

What comes next	Use	Example
A command or request (-(으)세요)	-(으)니까	추우니까 문을 닫으세요.
A suggestion (-(으)ㅂ시다, -자, “let’s” -아/어요)	-(으)니까	시간이 없으니까 택시를 타자.
A plain fact or result	-아/어서 (most common)	바빠서 못 갔어요.
Thanks or an apology	-아/어서	늦어서 미안해요.
A past reason	-아/어서 (no 었) -(으)니까 (었 OK)	늦어서 ✓ / 늦었어서 ✗ / 늦었으니까 ✓

Common mistakes

시간이 없어서 서두르세요.시간이 없으니까 서두르세요.

A command follows, so use -(으)니까.

어제 많이 걸었어서 다리가 아파요.어제 많이 걸어서 다리가 아파요.

No past tense before -아/어서.

선물을 줬으니까 고마워요.선물을 줘서 고마워요.

Thanks use -아/어서.

백화점에 가니까 옷을 샀어요. (가서 샀어요)백화점에 가서 옷을 샀어요.

“Went there and bought” is a sequence → -아/어서. -(으)니까 would sound like a discovery or a reason.

Mini practice · six questions

All sentences below are new. Try each one before opening the answer.

Fill in the blank날씨가 ______ 같이 산책할까요? (좋다)

Check answer

날씨가 좋으니까 같이 산책할까요?

A suggestion follows → -(으)니까.
Fill in the blank늦게 ______ 미안해요. (오다)

Check answer

늦게 와서 미안해요.

An apology → -아/어서: 와서.
Choose the better form감기에 (걸려서 / 걸렸으니까) 병원에 갔어요.

Check answer

걸려서

A plain statement about what happened → -아/어서.
Choose the correct form내일 일찍 출발해야 (해서 / 하니까) 오늘은 일찍 주무세요.

Check answer

하니까

A command follows (주무세요) → -(으)니까.
Find and fix the mistake숙제를 다 했어서 놀러 갔어요.

Check answer

숙제를 다 해서 놀러 갔어요.

No past tense before -아/어서: 해서.
Say it in KoreanSorry I couldn’t call you.

Check answer

전화 못 해서 미안해요.

Apology → -아/어서: 못 해서.

Your turn

Give one factual reason for something you did yesterday. Then use the same situation to make a suggestion with -(으)니까.

Questions Learners Often Ask

Can I say 늦었어서?

No. -아/어서 does not take the past marker 었 when it gives a reason. Put the tense at the end of the sentence instead: 늦어서 미안했어요. -(으)니까 is different: 늦었으니까 is correct.

Does -(으)니까 sound rude?

Not by itself, but it can sound firm, as if the reason should be obvious. When you politely explain your own situation, -아/어서 or -기 때문에 often sounds softer: 일이 있어서 못 가요.

How is -기 때문에 different?

-기 때문에 also gives a reason, but it sounds more formal and emphatic, and it is common in writing. Like -아/어서, it is not usually followed by a command or suggestion. You can learn it in Lesson 61.

Key point