If somewhere was dobre chłopcy it was incorrect.
Chłopcy is a plural for of chłopiec, masculine personal, so only possible adjective forms are also pl.masc-pers.: dobrzy chłopcy, młodzi chłopcy, starzy chłopcy, głupi chłopcy.
On the other hand, there is a word chłopak, masculine but not personal, its plural is chłopaki, and it takes not-masc-personal adjectives: dobre chłopaki, młode chłopaki, stare chłopaki, głupie chłopaki…
Simply the fact that it's a word for a person.
It only matters for male persons though, it doesn't change anything with female ones anyway.
I wouldn't say that "chłopak" is not personal, it's just a very weird exception that behaves in a very strange way.
"dobrzy" is the masculine personal plural and "dobre" is the non masculine personal plural (and the neuter singular). I believe it should have been "dobrzy chłopcy", though, are you sure it was "dobre chłopcy"?
So the forms are Dobry (masc sing), Dobra (femm sing), Dobrzy (masc plu), and Dobre (everything else)?
Dobre = feminine plural, Dobrzy = masculine plural, Dobra = feminine singular, Dobry = masculine singular
So...what are the adjective forms for masculine, plural nouns? I am very confused here.
It doesn't depend only on whether they're masculine, they have to be masculine personal - i.e. include at least one male person.
"Policjant" is masculine and "policjanci" are masculine personal, ergo: dobrzy policjanci
"Dom" is masculine but "domy" are not masculine-personal, ergo: dobre domy
but why "chłopaki" (all of them are masculine persons, aren't they?) are suddenly "dobre" as it written before?
special Slavic logic)
Same here. It's because we're not accustomed to the 'y' sound. Also, I tend to imagine any unfamiliar vowel as a variant of Serbian a, e, i, o, or u vowel (because I grew up hearing only them). However, after only 2 days of learning Polish, I started differentiating 'y' sound from very short 'e' sound.
Me too, but there must be a different explanation in my case as I am accustomed to the Polish 'y' sound.
One of the most common problems for the learners, perceiving the difference between E and Y... before Duolingo I would never think that someone could mistake them, they're so different for my Polish ear... But now I know that it's not that simple for others.