Actually, both are adjectives, Julian872323. See AspiroFremor's comment above for the correct declension of the adjective "dobry".
dobrzy is an adverb and does not change for male female or neuter, the way i understand and i think this is in nominative case. If I am wrong please correct.
adverb is "dobrze"- it does not change at all,- no cases no gender no number
dobrzy is nominative, plural, masculine personal form of adjective dobry
Not exactly. "Dobrzy" is "good" or "kind".
"nice" would be "mili" ("miły" in masculine singular).
Why not "dobri ludzie"? Or am I thinking too much in other slav languages? :-D
How would one say in Polish "good peoples" when referring to different sets of people. I guess the Italian la gente, la persona, le persone, il populo, and i populi are tweaking my learning. Then we have the American statement: "He is good people." I wonder how that would be translated into Polish?
I'm not sure what you mean by "different sets of people". Good peoples only refers to different nationalities/ethnicities and is therefore a morally highly questionable statement.
Regarding "he's good people", I'm convinced that people can be replaced by a person. That would convert it to standard English, without changing the meaning.