In short

Korean marks how sure you are and what your guess rests on. -(으)ㄴ/는/-(으)ㄹ 것 같다 is the all-purpose “I think / it seems” hedge; -나 보다 / -(으)ㄴ가 보다 says “apparently”, inferred from outside evidence; -(으)ㄹ 텐데 / -(으)ㄹ 테니까 adds a supposition with concern or with grounds. 비가 올 것 같아요 / 누가 왔나 봐요 / 바쁠 텐데 고마워요.

The rule

Step 1 - 것 같다: the everyday hedge, tensed on the modifier

This is just the modifier endings from lesson 24 plus the bound noun 것 (“thing”) plus 같다 (“to be like”). The tense and the word class live on the modifier, and 같다 normally stays present as 같아요:

Modifier form + 것 (space!) + 같아요

• 동사 now -> -는 것 같다: 가다 -> 가는 것 같아요 (ganeun geot gatayo, seems to be going).
• 동사 past -> -(으)ㄴ 것 같다: 먹다 -> 먹은 것 같아요 (meogeun geot gatayo, seems to have eaten).
• 동사 future / weak guess -> -(으)ㄹ 것 같다: 오다 -> 올 것 같아요 (ol geot gatayo, looks like it’ll come).
• 형용사 -> -(으)ㄴ 것 같다: 좋다 -> 좋은 것 같아요 (joeun geot gatayo, I think it’s good).
• 이다 -> -인 것 같다: 학생이다 -> 학생인 것 같아요 (haksaengin geot gatayo, seems to be a student).

Because 것 is a bound noun it keeps a space before it, and the following 같- tenses to [까], so 올 것 같아요 sounds like [올껃까타요]. This is also Korean’s favourite politeness hedge: a bare 좋아요 can sound blunt, so speakers soften even a firm opinion to 좋은 것 같아요 (I think it’s good).

Step 2 - -나 보다 / -(으)ㄴ가 보다: “apparently”, from outside evidence

Use this when you draw a conclusion from something you observe - a sound, a sight, someone’s face - not from inside knowledge:

동사 + -나 보다 ; 형용사·이다 + -(으)ㄴ가 보다 (auxiliary 보다 -> 봐요)

누가 왔나 봐요 (nuga wanna bwayo, someone must have come); 밖에 사람이 많은가 봐요 (bakke sarami maneunga bwayo, there must be a lot of people outside). Crucially it is not used about your own current state - you already know it. *저는 배가 고픈가 봐요 is wrong; just say 저는 배가 고파요. It fits yourself only when you work something out from evidence, as if from outside: 제가 많이 피곤했나 봐요 (I must have been really tired, said after nodding off). Anything built on 있다/없다 sides with the verbs here and takes -나 보다: 없다 -> 없나 봐요 (eomna bwayo), 재미있다 -> 재미있나 봐요 (jaemiinna bwayo) - the same class that keeps the verb-style -는 in front of 것 같다 (lesson 24).

Step 3 - -(으)ㄹ 텐데 / -(으)ㄹ 테니까: a supposition, plus concern or grounds

Both build on 터 (“expectation”) + 이다. -(으)ㄹ 텐데 floats a supposition and adds concern, regret, or contrast, and it often trails off:

Verb/Adjective stem + -(으)ㄹ 텐데 -> 많이 바쁠 텐데 도와줘서 고마워요 (you must be busy, yet thanks for helping).

-(으)ㄹ 테니까 offers the supposition (or your own intention) as grounds for what follows: 길이 막힐 테니까 지하철로 가요 (the roads will be jammed, so let’s take the subway); with a 1st-person subject it is a promise-as-grounds: 제가 준비할 테니까 걱정하지 마세요 (I’ll get it ready, so don’t worry).

Step 4 - The rest of the certainty scale

• -(으)ㄹ지도 모르다 = a low-probability “might, possibly”: 오후에 비가 올지도 몰라요 (ohue biga oljido mollayo, it might rain this afternoon). For a guess about something already finished, the past goes on the modifier exactly as with 것 같다: 이미 갔을지도 몰라요 (imi gasseuljido mollayo, they might already have left).
• -겠- (from lesson 25) is a snap inference from evidence right in front of you: 힘들겠어요 (himdeulgesseoyo, that must be hard).
• -(으)ㄴ 듯하다 is the formal / written twin of 것 같다: 비가 온 듯해요 (biga on deutaeyo, it appears to have rained).

Forms & patterns

것 같다 by word class and tense (tense sits on the modifier; 같아요 stays present):

-나 보다 / -(으)ㄴ가 보다 - what it attaches to:

Certainty scale, strongest to weakest:

Examples

Common mistakes

• 좋는 것 같아요 (jonneun geot gatayo) -> 좋은 것 같아요 (joeun geot gatayo) - 좋다 is an adjective, so its present modifier is -(으)ㄴ (좋은), never the verb’s -는. (But 있다-type adjectives keep -는: 재미있는 것 같아요.)
• 그 사람은 벌써 가는 것 같았어요 (meant as I think he has already left) -> 그 사람은 벌써 간 것 같아요 (Geu sarameun beolsseo gan geot gatayo) - a guess you make now about something finished puts the past on the modifier (간) and leaves 같다 present. 같았어요 is for an impression you had back then (it seemed he was leaving), not for a present guess.
• 저는 배가 고픈가 봐요 -> 저는 배가 고파요 (Jeoneun baega gopayo) - -(으)ㄴ가 보다 infers someone else’s state from evidence; you know your own hunger directly, so state it plainly.
• 날씨가 춥나 봐요 -> 날씨가 추운가 봐요 (Nalssiga chuunga bwayo) - 춥다 is a (ㅂ-irregular) adjective, so it takes -(으)ㄴ가 보다 -> 추운가, not the verb’s -나.
• 올것 같아요 (olgeot gatayo) -> 올 것 같아요 (ol geot gatayo) - 것 is a bound noun and always keeps a space before it, exactly like 할 수 있어요 and 갈 텐데.

Tips

• Tense lives on the modifier, class decides its shape. Choose 는/(으)ㄴ/(으)ㄹ for now/past/future, pick the verb or adjective column - then let 같아요 stay present. That one habit fixes most 것 같다 errors.
• 것 같다 is a courtesy, not just a guess. Koreans hedge firm opinions to sound modest: 좋은 것 같아요 is softer and more polite than a flat 좋아요. Overusing the blunt form can sound pushy.
• -나 보다 = “not my info.” Reserve it for what you deduce about others or the outside world. Not for feelings, hunger or pain you feel directly - those you simply report.
• 텐데 left hanging is sympathy. 힘드셨을 텐데… trailing off is a warm, complete-in-itself “you must have had a hard time…” - the concern lives in the unfinished ending.