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)

그리고 물어본 것보다 쓰셔서 감사합니다! 

설명 아주 길고 세세하게 쓰셔서 정말정말 감사합니다!! 이제 이해하는 것 같아요!