Reason (Noun)

때문에: because of, and the blame it carries

때문에 gives the reason for something. It's straightforward to attach and there are two things about it that catch people out: what it connects to, and how it sounds.

With a noun

Attach it directly. Nothing changes.

• 비 때문에 못 갔어요. (bi ttaemune mot gasseoyo) — "I couldn't go because of the rain."
• 너 때문에 늦었어. (neo ttaemune neujeosseo) — "I was late because of you."

With a verb, you need 기

This is the mistake worth fixing early. 때문에 can't attach straight to a verb — you have to turn the verb into a noun first with 기.

• 비가 오기 때문에 못 갔어요. (bi-ga ogi ttaemune mot gasseoyo) — "Because it was raining, I couldn't go."
• 바쁘기 때문에 못 가요. (bappeugi ttaemune mot gayo) — "I can't go because I'm busy."

So: noun + 때문에, but verb stem + 기 때문에. Skipping the 기 is one of the most common intermediate errors.

It sounds like blame

This is the part that isn't in most explanations, and it matters socially.

때문에 points a finger. 너 때문에 (neo ttaemune, "because of you") is an accusation, not a neutral statement of cause. Used about a person, it almost always carries fault.

When the cause is positive, Korean switches particles entirely and uses 덕분에 (deokbune, "thanks to"):

• 너 때문에 늦었어. — "I was late because of you." Your fault.
• 네 덕분에 성공했어. (ne deokbune seonggonghaesseo) — "I succeeded thanks to you." Your credit.

Same grammatical slot, opposite emotional load. Using 때문에 for something good sounds strange, and using 덕분에 for something bad sounds sarcastic.

때문에 vs 아/어서 vs (으)니까

Three ways to say "because", and they aren't interchangeable.

• 때문에 — states a cause, fairly formal, and can assign blame.
• 아/어서** — the everyday "so", neutral. Can't be used with commands or suggestions.
• (으)니까** — reason plus judgment, and it's the one you use before a command.

• 비가 오니까 우산을 가져가세요. (bi-ga onikka usan-eul gajyeogaseyo) — "It's raining, so take an umbrella." ✓
• 비가 와서 우산을 가져가세요. ✗

That restriction on 아/어서 is the practical reason to learn all three rather than picking a favorite.