Sometimes the adjective goes before the noun and sometimes after the noun in Portuguese. Is there any way to tell when it should go before or after?
Hobby is translated as passatempo favorito, but when I typed "jogar cartas é um passatempo favorito popular" Duo said I was wrong?
Hobby is just passatempo
Passatempo favorito is favorite hobby.
What is the difference between "bicampeão" and "campeão" ?
A "bicampeão" has become a "campeão" twice. You'll sometimes see "bicampeão" translated as "two-time champion".
What made you ask that question here?
It was one of the options for the choice out of three sentences. I had already had the word "campeão" and was curious about the difference.
I see that "Jogando cartas é um passatempo popular." Is incorrect.
Can someone give/explain briefly the rule why?
Yes, it is incorrect. When a verb works as the subject of a sentence, in Portuguese you should use infinitive.
Is "um" really necessary before passatempo? I understand that it is the literal translation, but this strikes me as a case where it is implied and not necessary...maybe that comes from my knowledge of Spanish.