Hello:
I was hoping someone could clarify the genitive case of nouns. I understand that it means possessive, but I don’t understand why it seems to change.
Ex. pretty girl of mine = bella puella meae, or bella puellae meae, or bellae puellae meae?
Meus is the singular form. The plural form is noster. Furthermore, these are adjectives. The -ae and -arum endings are for nouns and adjectives of the first declension, with the former being singular and the latter plural.
Note that meus denotes possession whether used as an adjective or as a pronoun. In this case it is used as an adjective, just as bellum (1) is used. So it will agree with the thing it describes (and posseses) in gender, number, and case.
One pretty girl: puella bella; two pretty girls: puellae bellae.
My pretty girl: puella bella mea; my pretty girls: puellae bellae meae.
Your (2) pretty girl: puella bella tua; Your pretty girls: puellae bellae tuae.
The adjective agrees with the thing possessed. Also note that “girls of mine” and “my girls” are equivalent, just as patria mea is “my fatherland” or “fatherland of me”
The genitive case would be describing a possession of your pretty girl, say a crown: corona puellae bellae tuae.
So you would not say::
bella puella meae, because meae doesn’t agree in case with puella.
bella puellae meae doesn’t have bella and puellae in agreement.
bellae puellae meae is right, they all agree.
Now, puellarum mearum is “of my girls” (as in “the crowns of my girls”)
puellas mearum doesn’t agree in case
puellae mearum doesn’t agree in case either.
HTH
(1) I have to admit that I thought this was a warlike girl at first. Pulchra is the word that springs to mind for pretty.
(2) This is singular. When referring to more than one “you”, as in Girls, your crowns are pretty.” vester is the adjective to use.