Reason with -(으)니까 in Korean

Overview

The grammatical ending -(으)니까 is one of the most common ways to express reason or cause in Korean. At the CEFR A2 level, learning this pattern will significantly expand your ability to explain why something happens, justify a suggestion, or give a reason for a request.

What makes -(으)니까 special compared to the other major reason marker -아/어서 is its assertive, speaker-centered quality. When you use -(으)니까, you are presenting your own judgment or experience as the basis for a conclusion. Crucially, it can be followed by imperative sentences (commands), propositive sentences (suggestions), and requests — something -아/어서 cannot do.

You will encounter -(으)니까 constantly in everyday Korean, from casual conversations to polite requests. It is the go-to connector when you want to say "since" or "because" before telling someone what to do or proposing a plan.

Formation / How It Works

The form you attach depends on whether the verb or adjective stem ends in a consonant or a vowel.

Attachment Rules

Stem ending	Attach	Example
Vowel	-니까	가다 → 가 + 니까 → 가니까
Consonant	-으니까	먹다 → 먹 + 으니까 → 먹으니까
ㄹ consonant	-니까 (drop ㄹ)	멀다 → 머 + 니까 → 머니까

Past Tense

To express a reason that happened in the past, attach -(으)니까 to the past tense stem:

Type	Formation	Example
Past	stem + 았/었 + 으니까	먹었으니까 (since [I/you] ate)

Comparison: -(으)니까 vs. -아/어서

Feature	-(으)니까	-아/어서
Imperative (commands)	Yes	No
Propositive (let's...)	Yes	No
Past tense in reason clause	Yes	No
Speaker's subjective judgment	Strong	Neutral
Tone	Assertive, justifying	Explanatory, sequential

Examples in Context

Korean	English	Note
비가 오니까 우산을 가져가세요.	It's raining, so take an umbrella.	Imperative follows reason
시간이 없으니까 빨리 가요.	We don't have time, so let's go quickly.	Propositive sentence
배가 고프니까 먹을까요?	I'm hungry, so shall we eat?	Suggestion with -(으)ㄹ까요
추우니까 문 닫아.	It's cold, so close the door.	Casual imperative
피곤하니까 좀 쉴게요.	I'm tired, so I'll rest a bit.	Speaker's own state as reason
내일 시험이 있으니까 오늘 공부하세요.	There's an exam tomorrow, so study today.	Advice/command
늦었으니까 택시를 탑시다.	It's late, so let's take a taxi.	Past tense + propositive
맛있으니까 드셔 보세요.	It's delicious, so please try it.	Polite recommendation
비싸니까 다른 데 가자.	It's expensive, so let's go somewhere else.	Casual suggestion
이미 했으니까 걱정하지 마세요.	I already did it, so don't worry.	Past reason + negative imperative

Common Mistakes

Wrong: 비가 오아서 우산을 가져가세요. Right: 비가 오니까 우산을 가져가세요. Why: You cannot use -아/어서 before an imperative (command). When the second clause is a command or suggestion, you must use -(으)니까.

Wrong: 먹다니까 Right: 먹으니까 Why: When the stem ends in a consonant (먹), you need the connecting vowel 으 before 니까. Attaching 니까 directly to the dictionary form is incorrect.

Wrong: 갔아서 괜찮아요. Right: 갔으니까 괜찮아요. Why: When expressing a past reason with a subjective conclusion, -(으)니까 is the natural choice. Using -아/어서 with past tense in the reason clause sounds unnatural in Korean.

Wrong: 멀으니까 Right: 머니까 Why: When the stem ends in ㄹ (멀다), the ㄹ is dropped before adding -니까. This is a standard ㄹ-irregular rule.

Usage Notes

In spoken Korean, -(으)니까 is often shortened to -(으)니깐 or even -니까는 in very casual speech. You may also hear -니까요 as a sentence-ending form when the speaker is explaining their reason and seeking understanding from the listener (e.g., 바쁘니까요 — "Because I'm busy, you see"). This sentence-ending use is common when someone feels the need to justify themselves.

The assertive tone of -(으)니까 means it can sound slightly forceful. In very polite or formal situations, speakers sometimes soften the overall sentence with polite endings or hedging expressions. Among close friends, this directness is perfectly natural and expected.

Regional variation is minimal for this grammar point — -(으)니까 is used consistently across Korean dialects, though the casual shortened forms may differ slightly.

It is worth noting that -(으)니까 can also express a discovered reason or realization: 가 보니까 사람이 많았어요 (When I went, I found there were many people). In this temporal-discovery use, it overlaps somewhat with -(으)니 (a shorter form). At the A2 level, focus on the causal "because/since" use first, but be aware you will encounter this extended meaning as you progress.

Practice Tips

• Pair with commands and suggestions. Every time you want to tell someone to do something, practice building the reason with -(으)니까 first. For example: "It's hot, so open the window" → 더우니까 창문 열어.
• Swap with -아/어서 to feel the difference. Take the same reason and try it with both connectors. Notice which second clauses sound natural with each. This will build your intuition for when to choose -(으)니까.
• Practice past tense reasons. Combine past tense stems with -(으)니까 to talk about completed reasons: 다 먹었으니까 갑시다 (We've finished eating, so let's go).

Related Concepts

• Reason with -아/어서