Core Pattern Rule: Guessing with -나 봐 / -(으)ㄴ가 봐
This pattern expresses the speaker's assumption or guess about a situation, based on some observed evidence. It's equivalent to saying "it seems like," "I guess," or "it looks like" in English.
The ending changes depending on what it's attached to.
Action Verbs: -나 봐
Attach -나 봐 to the stem of action verbs for present tense guesses. For the past tense, use -았/었나 봐.
Present: 오다 → 오나 봐 (It seems to be raining.)
Past: 고장 나다 → 고장 났나 봐 (I guess it broke.)
Descriptive Verbs (Adjectives) & 이다: -(으)ㄴ가 봐
Attach -(으)ㄴ가 봐 to the stem of descriptive verbs.
Vowel stem: -ㄴ가 봐 (e.g., 배고프다 → 배고픈가 봐)
Consonant stem: -은가 봐 (e.g., 좋다 → 좋은가 봐)
Nouns: Use (noun)인가 봐 (e.g., 맛집 → 맛집인가 봐)
있다 / 없다
These two words behave like action verbs and take -나 봐.
있다 → 있나 봐
없다 → 없나 봐 (e.g., 아무도 없나 봐)
비가 오나 봐. / 배고픈가 봐. / 기계가 고장 났나 봐. / 맛집인가 봐.
Synonyms & Differences: -나 보다 vs. -는 것 같다
While both patterns express conjecture, they have different nuances.
-나 보다 / -(으)ㄴ가 보다
This form implies that the speaker has just come to a conclusion based on new, direct evidence. It carries a sense of realization or discovery, like "Oh, I see now..." or "Ah, so it seems..."
Use when: You see dark clouds and say, 비가 오나 봐. (It looks like it's going to rain.)
-(으)ㄹ/는/(으)ㄴ 것 같다
This is a more general and versatile pattern for expressing an opinion, thought, or guess. It can be based on direct evidence, but it can also be a subjective feeling or something you heard indirectly. It's a neutral "I think..." or "it seems..."
Use when: You could say 비가 오는 것 같아요 based on seeing clouds, but also based on a weather forecast you read earlier.
In short, -나 보다 is for guesses triggered by immediate observation, while -는 것 같다 is a broader way to say "I think."