Here are differences 1) -은/는/을 것 같다, -은/는/을 듯싶다/듯하다 (low certainty) ① no base, vague supposition, low certainty ② subjective personal opinion. ③ Used for humble supposition ④ Not commonly used in literary. Commonly used in colloquially e.g. 제 생각에는(personal opinion) 차가 막힐 것 같아요. 왠지(no base) 차가 막힐 것 같아요. 2) -나/은가 보다, -은/는/을 모양이다 (middle certainty) ① Prompt supposition based on direct perceived situation ② Not used for subjective personal opinion e.g. 사람들이 우산을 쓴 걸 보니(based on perceived situation) 비가 오나 봐요(present assumption) 3) -겠다 (high certainty) ① Prompt supposition based on direct perceived situation ② Supposition of future event (Supposition of past event with '-었겠-') ③ Used for subjective personal opinion ④ Used for expressing sympathy with the listener(힘들었겠다...) e.g. 먹구름이 낀 걸 보니(based on perceived situation) 비가 오겠어요.(future assumption) 4) -을걸, -을 것이다. -을 거예요(high certainty) ① Logical supposition based on past experience ② Not used for subjective personal opinion. ③ Used for reassuring listener(잘 될 거야) e.g. 현정이는 아마 결혼을 했을 거예요. 전에 남편 얘기를 들었거든요.(based on past experience) 1. If I hear a baby crying 무서운가 봐요(present assumption) is better 2. If I see a long line in a movie theatre 재미있나 봐요(present assumption) is better 3. If my classmate comes in late 늦게 잤나 봐(present assumption) is better

If you tries to understand -겠다 or -나 보다 by English, there would be a lot of meanings to know. So, it is impossible to conclude that -겠다 is an instant response or -나 보다 is an supposition based on behavior or something. What I'm trying to say is that : your explanation is not wrong; but it is not right either. What you have to do is to understand Korean as Korean, not English, though it is really hard to do. For example, if a Korean asked you about the meaning of GOOD, you can say 좋다 or 좋은, but Korean word 좋다 or 좋은 can't contain all the meaning of GOOD - such as goods as product or something. That Korean is same as you right now. Your explanation is not wrong, since we can translate good as 좋은. But, if you limit -겠다's or -나 보다's meaning as your explanation above, I rather say it is wrong. For now, I'll answer you just focusing on the examples you listed. 1. If I hear a baby crying, is 무서운가 봐요 (based on behavior) better than 무섭겠다 (instant response)? (아기가) (eg. 인형이) 무서운가봐요 is correct. (eg. 인형이) 무섭겠다 can be said in this situation : baby is not crying now, you saw something that you think the baby might get frightened and gonna cry. 2. If I see a long line in a movie theatre, is 재미있나 봐요 better than 재미있겠다? Both can be correct, but the specific meaning differs. 재미있나 봐요 focuses on the movie. It is used when you think the movie might be 재미있다 since there is a long line to watch that movie. But, 재미있겠다 focuses on yourself. It is used when you think you can have 재미 if you watch that movie. So, if someone say 재미있나 봐요, he/she maybe not gonna watch what movie. But, if someone say 재미있겠다, he/she is more likely to watch what movie 3. If my classmate comes in late, is 늦게 잤겠다 better than 늦게 잤나 봐? 늦게 잤나봐? is more likely to be used. 늦게 잤겠다 can be used in : you talked to your classmate late at night. comes late or not is not that important with this expressions.