I think my brain has short-circuited, because I cannot figure out why, in the following sentence, the adjectives are in the nominative. "Fīs lēnior et melior, accēdente senectūte?" I thought the only way that such could happen is if they were the object of 'sum'. Is it because they're modifying the subject ('You') of the verb?
Fieri, "to become", takes the nominative in the subject and predicate, just like esse. After all, it's technically a passive verb.
Such happens with copular verbs, because those verbs do not have objects. Sum is the main copula in Latin.