The nominative case is used for a subject, and the accusative for an object. Now there are four more cases for you to look at. First, the genitive case. The English preposition of is used to translate the genitive case:

dominus puellae

the master of the girl (the girl’s master)

fīlia servī

the daughter of the slave (the slave’s daughter)

girl

slave

Nominative

puella

servus

Accusative

puellam

servum

Genitive

puellae

servī