to think = to be of the opinion that to suppose = to make a decision based on evidence to assume = to make a decision without evidence but rather on personal predilection They're all quite similar, and you could use them fairly interchangeably, especially the first two. Only use "assume" when you're making a decision without evidence but rather on what you think to be true.

'think' is the mental process you go through pondering upon an issue, a problem and thus form your idea about it or come to a conclusion that you are certain of and convinced of it to be true ,you will express that as : 'I think that it is A.' 'suppose' Your idea or end result of choosing the problem's solution to be 'A' is not necessarily built on previous mental thinking ,but it depends rather on information you already have about the issue, your experience or knowledge or all of them ,upon which you form your idea, that you probably with a certain degree of certainty believe to be true ,but there remains some elements of doubt and it is not a 100% conviction of your result, it is like saying : "Built on all my experience and the beforehand evidence I think that it must be A ,but I am not 100% sure though ." "I suppose it is A." 'assume': Here you build your assumption that 'A' is the correct choice, while you don't really have any proof, you just believe it is true .Here your choice of 'A' is a hypothesis that you take for granted ,while you have no evidence to prove it. You don't by any means confirm any certainty of your choice, but just express your belief in it . "I assume it is A'

I think the answer is "not a lot"! As a native speaker I was trying to think of ways in which we use them differently. There is one: you couldn't put "assume" or "suppose" for think in the last sentence. However I would think/imagine/suppose/assume/guess[American English] you mean in a situation where you are saying something is true. Assume has more certainity about it for example: "I assume you have done the work I asked you to do" is more definite, the expectation is that you actually have done so. However, all of this words can take on more certainty when stressed: I IMAGINED you had cleared up! What a mess you have left!

I see. Thank you! You mean that even if there are more than two things to compare, I should use "between" instead of "among", right?

you should ask: 'what is the difference between...' It is difficult to explain what 'among' means. To be in a group, to be one of a number of things... 'Among my classmates, Petra is the best.' 'It was so dark I couldn't see among the trees' Hm.