-구나/-는구나

A plain (반말) ending for a fresh realization or mild surprise: 'Oh, so…!'. Used to friends, younger people and children, or when talking to oneself.

문장 끝·말투Endings & speech style

Han: -구나 is 반말; even with -시- it is not polite enough when speaking to a teacher. Use -군요 or -네요.

Examples

너도 이 노래를 좋아하는구나!So you like this song too!

벌써 시간이 이렇게 됐구나.Oh, it's already this late.

그동안 많이 힘들었겠구나. 오늘은 푹 쉬어.It must have been really hard for you. Get a good rest today.

Don’t mix up

-구나/-는구나vs-군요/-는군요-군요 is the polite counterpart: 많이 컸구나 (to a child) vs 아드님이 많이 컸군요 (to a parent).

-구나/-는구나vs-네-네 reacts to what you see right now; -구나 sounds more like grasping a fact or its meaning: 비가 오네 'oh, it's raining' vs 그래서 늦었구나 'so that's why you were late'.

How to build it

V+는구나

A+구나

N+(이)구나 / past -았/었구나 / guess -겠구나

오다 → 오는구나; 먹다 → 먹는구나; 예쁘다 → 예쁘구나; 맛있다 → 맛있구나; 학생 → 학생이구나; 갔다 → 갔구나; 힘들다 → 힘들겠구나

Present action verbs take 는구나 (먹는구나); adjectives, 있다/없다 and past or -겠- forms take 구나 (예쁘구나, 맛있구나, 갔구나).
It is 반말: say it to friends, juniors and children, or to yourself (벌써 봄이구나). To elders and in polite speech use -군요 or -네요.
It is typical of parents and teachers talking to children, and of inner monologue in novels and dramas.

Classic mistake

Wrong: ~~선생님, 한국어를 정말 잘하시는구나!~~

Right: 선생님, 한국어를 정말 잘하시는군요!

-구나 is 반말; even with -시- it is not polite enough when speaking to a teacher. Use -군요 or -네요.

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