In latin you attach ending to words depending on if it is the subject, with a predicate, direct object, indirect object, vocative, ect.

For instance, puella is nominative for "girl."
puellae is "of the girls."

however, puellae can also mean "girls."

Voco puellam. I call to the girl. -am being the ending for the direct object. : )

Puella is also the first case for nouns. It changes if it is of a different case, such as:

Gaium agricolae video.

I see Gaius in the fields. -um is the direct object ending for that case of nouns. in this case, masculine.

Entendes? ( Is so excited to be talking about languages! did I say that right in spanish? )