In short

Korean has three main “because” connectors and they are not interchangeable. -아/어서 is the everyday, neutral one (built on the 아/어 form) - but it never carries a tense marker and never lets a command or suggestion follow. -(으)니까 foregrounds the reason and does allow a past tense (늦었으니까, since it’s late) and a following command (비 오니까 우산 가져가세요, it’s raining, so take an umbrella). -기 때문에 (and N + 때문에) is the formal, written choice.

The rule

Step 1 - -아/어서: cause, and “and then” (no tense, no command)

Take the 아/어 form you already build for 해요체 (lesson 10) and add 서 instead of 요: 가다 -> 가서 (gaseo), 먹다 -> 먹어서 (meogeoseo), 하다 -> 해서 (haeseo). It has two jobs:

• Cause - because: 배가 아파서 못 왔어요 (baega apaseo mot wasseoyo, I couldn’t come because my stomach hurt).
• Sequence - and then, with the same subject carried into the next clause: 학교에 가서 공부했어요 (hakgyoe gaseo gongbuhaesseoyo, I went to school and studied there). Here 서 links one actor’s two connected actions - a different subject in the second clause is wrong.

Two hard limits define -아/어서: the tense goes only on the final verb (never *갔어서), and no imperative or propositive may follow it (no *-(으)세요, no *-(으)ㄹ까요? after 서).

Step 2 - -(으)니까: reason with tense and commands allowed

-(으)니까 attaches straight to the stem with the familiar vowel-vs-받침 split: 가다 -> 가니까 (ganikka), 먹다 -> 먹으니까 (meogeunikka); a ㄹ-stem drops its ㄹ, 살다 -> 사니까 (sanikka); and a ㅂ, ㄷ or ㅅ stem changes class first, exactly as before any other (으) ending (lesson 17): 덥다 -> 더우니까 (deounikka), 걷다 -> 걸으니까 (georeunikka), 짓다 -> 지으니까 (jieunikka). Unlike 서 it accepts everything 서 refuses:

• Tense before it: 늦었으니까 서두르세요 (neujeosseunikka seodureuseyo, we’re late, so hurry).
• A command or suggestion after it: 비 오니까 우산 가져가세요 (bi onikka usan gajeogaseyo), 피곤하니까 좀 쉴까요? (pigonhanikka jom swilkkayo?, since we’re tired, shall we rest?).
• Discovery / “upon doing”: put -(으)니까 after a verb and the second clause states what you then noticed: 가 보니까 아무도 없었어요 (ga bonikka amudo eopseosseoyo, when I went and looked, no one was there).

Step 3 - -기 때문에 / N 때문에: the formal, written reason

Nominalize the clause with -기 (lesson 36) and add 때문에: 비가 오기 때문에 경기가 취소됐어요 (biga ogi ttaemune gyeonggiga chwisodwaesseoyo, the match was cancelled because it’s raining). After a bare noun use 때문에 alone: 시험 때문에 바빠요 (siheom ttaemune bappayo, I’m busy because of exams). To make the reason explicit, front it with 왜냐하면 and close with -기 때문이다/이에요: 왜냐하면 일이 많기 때문이에요 (waenyahamyeon iri manki ttaemunieyo, (it’s) because I have a lot of work). This set is formal and common in writing and announcements.

Step 4 - The sentence-adverb equivalents

When the cause is a whole previous sentence, restart with an adverb instead of a connector: 그래서 (geuraeseo, = 서), 그러니까 (geureonikka, = -(으)니까), 그 때문에 / 그렇기 때문에 (geu ttaemune / geureoki ttaemune, = 때문에). 비가 와요. 그래서 안 나가요. (Biga wayo. Geuraeseo an nagayo. It’s raining. So I’m not going out.)

Step 5 - The fixed exception: thanks and apologies take -아/어서

With 미안하다 / 죄송하다 / 고맙다 / 감사하다 the reason is always joined with -아/어서, never -(으)니까: 늦어서 죄송해요 (neujeoseo joesonghaeyo, sorry for being late), 도와줘서 고마워요 (dowajwoseo gomawoyo, thanks for helping). Memorize this as a fixed frame.

Forms & patterns

-아/어서 - the 아/어 form plus 서:

-(으)니까 - by the last sound of the stem:

The decision table - which connector fits:

Examples

Common mistakes

• 늦었어서 죄송해요. (Neujeosseoseo joesonghaeyo.) -> 늦어서 죄송해요. (Neujeoseo joesonghaeyo.) - -아/어서 never takes a tense marker; the tense lives only on the final verb.
• 늦어서 미안하세요. (Neujeoseo mianhaseyo.) -> 늦어서 미안해요. (Neujeoseo mianhaeyo.) - 미안하다 describes your own feeling, so you cannot add the honorific -(으)세요 to yourself.
• 비가 와서 우산 가져가세요. (Biga waseo usan gajeogaseyo.) -> 비가 오니까 우산 가져가세요. (Biga onikka usan gajeogaseyo.) - a command cannot follow -아/어서; use -(으)니까 when an imperative or suggestion comes next.
• 도와주니까 고마워요. (Dowajunikka gomawoyo.) -> 도와줘서 고마워요. (Dowajwoseo gomawoyo.) - thanks and apologies fix -아/어서 as their connector; -(으)니까 does not fit here.
• 시험 기 때문에 바빠요. (Siheom gi ttaemune bappayo.) -> 시험 때문에 바빠요. (Siheom ttaemune bappayo.) - after a plain noun use bare 때문에; -기 때문에 attaches only to a verb or adjective stem.

Tips

• The two-question filter. Does a command or suggestion follow, or do you need a past tense on the reason? If yes to either, -아/어서 is out - reach for -(으)니까. -아/어서 is tenseless and statement-only.
• Register ladder for “because”: -아/어서 (neutral, softest, most common in speech) < -(으)니까 (foregrounds the cause, more assertive) < -기 때문에 (formal, written). 왜냐하면 … -기 때문이다 is the explicit “the reason is that …” frame.
• -(으)니까 also does discovery. 가 보니까, 먹어 보니까 = on doing X, I found that …. The second clause is what you noticed, not an order.
• Lock in the fixed pair. 미안하다 / 죄송하다 / 고맙다 / 감사하다 always take -아/어서: 늦어서 죄송해요, 와 줘서 고마워요. Saying *늦으니까 미안해요 sounds off to a native ear.