Latin/Agreement of Verb
I'm having some trouble figuring out a question on my homework and I was hoping you could help explain it.
"Gallī numerō Rōmānīs superiorēs erant, tamen victī sunt."
"Why does 'victī' have the masculine plural ending?"
Because the subject "Galli" is in the masculine plural. The participle part ("vincti") of the full verb form ("vincti sunt") must agree in gender and number, in the nominative (subject) case, with the subject of the sentence.
As you have probably learned by now in your studies, the perfect passive indicative forms (present perfect, past perfect, and future perfect) of verbs are formed by the perfect passive participle (here "vincti") and a form of the verb "esse" (here "sunt").
Since the participle part is a verbal adjective, it has the adjectival forms of a first/second-declension adjective (-us, -a, -um) and agrees with its subject (which it modifies) in gender and number in the nominative (subject) case.