puella
the girl (subject)
puellam
the girl (object)
puellae
to/for the girl; the girls (subject)
puellas
the girls (object)
puellis
to/for the girls
servus
the slave (subject)
servum
the slave (object)
servo
to/for the slave
servi
the slaves (subject)
servos
the slaves (object)
servis
to/for the slaves
mercator
the merchant (subject)
mercatorem
the merchant (object)
mercatori
to/for the merchant
mercatores
the merchants (subject or object)
mercatoribus
to/for the merchants
nominative
subject/the one doing the verb
accusative
object/translate after the verb
dative
to/for the noun
singular
one of a thing
plural
more than one
First declension
nouns with endings like puella
Second declension
nouns with endings like servus
Third declension
nouns with endings like mercator