Core Grammar Rule: -(으)니까
-(으)니까 is a connective ending used to express a reason or cause for the following clause. It's often used when the reason is a logical basis for a command, suggestion, or a statement of the speaker's will.
Meaning & Usage
Meaning: It translates to "because," "since," or "so." It connects two clauses, where the first clause [Reason] is the cause for the second clause [Result/Action]
When to use: Use it when the reason in the first clause leads to a suggestion (~하자/~합시다), a command (제발/~하세요), or a statement of intention (~합니다/~할게요). It can also be used to state a reason for a fact that has already occurred.
Conjugation Rules
Rule: The form depends on whether the verb or adjective stem ends in a vowel or a consonant.
Vowel or ㄹ: Attach ~니까 directly to the stem. For stems ending in ㄹ, the ㄹ is dropped. (e.g., 가다 → 가니까 / 놀다 → 노니까)
Consonant: Attach ~으니까` to the stem. (e.g., 먹다 → 먹으니까 / 좋다 → 좋으니까)
Vowel/ㄹ: ~니까 / Consonant: ~으니까
Synonyms & Differences: -(으)니까 vs. -아/어서
While both -(으)니까 and -아/어서 mean "because" or "so," they are not always interchangeable. The key difference lies in the type of sentence that can follow.
Commands & Suggestions
Context: When the main clause is a command (e.g., ~(으)세요) or a suggestion (e.g., ~(으)ㅂ시다, ~(으)ㄹ까요?), you must use -(으)니까. Using -아/어서 in this context is grammatically incorrect.
Example: 날씨가 좋으니까 공원에 갑시다. (Let's go to the park since the weather is good.) (O) / 날씨가 좋아서 공원에 갑시다. (X)
Expressing Gratitude or Apology
Context: When expressing a reason for gratitude or an apology, -아/어서 is the natural choice. Using -(으)니까 in these situations can sound unnatural or even slightly arrogant, as if you're justifying your reason for being thanked or for apologizing.
Example: 도와주셔서 감사합니다. (Thank you for helping me.) (O) / 늦어서 죄송합니다. (I'm sorry for being late.) (O) / 늦었으니까 죄송합니다. (X)
`-(으)니까`: Use with commands/suggestions / `-아/어서`: Use for general reasons & thanks/apologies