I suppose you don't know him

i guess you don't know who he is.

придаточное предложение не должно быть в вопросительной форме

I guess doesn’t mean the same as I think/believe. The latter indicate that you’re not sure, where as the former can be paraphrased as “Judging from what I see/hear, it’s apparent/clear/obvious that…”

Uly, the original text says «я полагаю». Is the best equivalent for it ‘I suppose’? At least dictionaries directly suggest this translation

I suppose and I guess mean the same thing - different from I think and I believe.

Russians misuse these all the time.

Yes, it’s just because practically there’s no difference in Russian between “я думаю, я полагаю”, while “мне кажется” directly implies that the speaker is not quite sure

Aha

Угу