dobry/dobra/dobre/dobrzy are adjective forms = good
They describe nouns
dobry is for masculine singular
dobra - feminine singular
dobre- neuter singular and not masculine personal plural (one)
dobrzy- plural masculine personal (oni)
dobrze - is adverb= well. It describes verbs.
Why is it not "dobra kanapki" if kanapka is feminine? Is it because kanapki is plural so the adjective changes?
As far as I understand, you are right. Dobra kanapka (sg.), dobre kanapki (pl.).
Even in this table you see that there are at least four possibilities to form plural nominative: -y, -ki, -gi or -e. To really answer which to choose one must use whole ending (last syllable, or more).
y: śluza -> śluzy, meduza->meduzy, kapuza ->kapuzy
ki: kanapka -> kanapki, matka -> matki, wariatka -> wariatki, półka -> półki
gi: pręga -> pręgi, uwaga -> uwagi, ranga -> rangi, plaga -> plagi