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

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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"

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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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