"myśleć" is literally "thinking", so every time you would use "I think that[…]" in English, you would use "Myślę, że[…]" in Polish.
"uważać" is pretty close to consider, other popular "opinion indicators" in Polish are "Moim zdaniem" - "In my opinion", "sądzić" - literally "judging" but also used to state opinion("Sądzisz, że on jest dobry?") and quite a few of others less popular(I think).
This could also be "Do you consider it good", talking about a grammatically masculine object, right?
No, it's a question about an opinion, I think I would simply use "czy wiesz" for your version.
Well, 'kind' is not such an exact translation of 'dobry', but in this context I guess it works, so we added your version.
Are you considering, that he is good?
and the 2nd questions - why is there a him in your translation as the version in polish is on?
You could also say "Czy uważasz go za dobrego człowieka?" = "Do you consider him a good person?"
I really don't think any English native would use continous form "considering" in such a context. You consider something generally, not at the moment.
Different ways of conveying the same message.
"Do you consider that he is good?" works as well, with "he" being a very direct equivalent of "on".
Polish, as you know, has a lot of cases, English almost doesn't, but he->him is a reminiscence of the times when it had cases as well. But usually you won't find such situations with different English cases, so nothing will make you wonder about it. I understand that it's a little confusing here, but both versions in English are correct, both versions in Polish are correct, it's just the matter of which one is a bit more natural and probable in usage.
But would it really mean that it's your opinion? Or rather that you are right now thinking and wondering about whether he is in fact a good person? Same with your other comment, "Have you considered" - that won't work because of a wrong tense, but anyway, doesn't it also mean "Have you given it a thought that maybe he's a good person"? The Polish sentence asks about your opinion.
Suppose not, but I definitely would not use that sentence the way it is structured. I would probably say something like "Do you consider that he is good?" or "Do you consider him to be good?" Probably the latter rather than the former.
"Do you consider that he is good?" works, "Do you consider him to be good?" is ex aequo 'the best answer'.
I will now make it the only 'best answer', especially that it's the 'original sentence' in the ENG->PL version of this exercise.
If "że" starts a new clause (and it usually does), it has to be preceded with a comma.