"To some we bring pears at lunch, to others not."
Translation:Unora le aducem pere la prânz, altora nu.
Is it not correct to also say, Unui aducem pere la prânz, alții nu?
I see. It's not "unui" and "alții" here because those would be the nominative forms. We need the dative here, because we are giving pears "to some" (dative) but not "to others" (dative).
my take on it (no expert): The forms with definite article (unul instead of un) end in -a, so maybe "Unuia îi ..., altuia nu" if the recipient was in singular, but here they are in plural.
And "alții" would be the wrong case, it is accusative like in "accuse/see others" not dative "give (to) others"
nominativ-acuzativ un/o - unii/unele
genitiv-dativ unui/unei - unor
unul (the one)
nominativ-acuzativ unul/una - unii/unele
genitiv-dativ unuia/uneia -