What is the difference between "guess" and "think." And how do you use these words?

Question

Answer

The word "guess" has the idea of imagine and "think" has the idea of having an opinion. Examples: 
I guess you're right =  I imagine/suppose you're right 
 I think you're right  = I've thought about it and my opinion is that I agree with you.

"I guess" is  often used in American English. Australians say  "I reckon"

Answer

I guess I'll have to think about it.

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In a lot of contexts I tend to think of "think" as usually more strong.  "Guess" is more of an after though and softens the amount of confidence being given to what is claimed

I think so. - It's probably true, but I'm not 100% certain.
I guess so. - I don't know, maybe it's true.

I think the Raiders will win. - I probably have some kind of knowledge that makes me think they have some kind of advantage.
I guess the Raiders will win. - I might not know anything about football at all, but want to pick a winner anyways.

Answer

I think --------> Giving opinion 
= In my opinion

I guess -----------> Giving opinion
= I'm not certain / I'm not sure

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