"Corinna is a girl."
Translation:Corinna est puella.
There is no object in the sentence, and therefore no 'puellam' (in the accusative). Corinna is the subject and puella restates/describes that subject, therefore is also the subject (and remains in the nominative).
Corinna est puella. Puella est Corinna.
I've heard this scenario referred to as 'nominative complement' in studying Latin, and 'predicate nominative' more generally.