When you are trying to translate a Latin noun into English or English into Latin, you should know which of the five declensions the noun falls into. If you know the declension and the dictionary forms of a noun, you're set. For instance, the word puella, a first declension word that will be listed as "puella, -ae, f." or something similar in the dictionary, is feminine (that's what the "f." stands for; m. stands for masculine and n. stands for neuter) and is first declension, as you can tell from the second part of the dictionary listing, here; "-ae".
The genitive (cāsus patricus 'paternal case' in Latin) is the name for this second form ("-ae" for the first declension) and is easy to remember as the equivalent of a possessive or apostrophe-s case in English. That's not its complete role, though. In Latin, the genitive is the case of description. The use of one genitive noun limits the meaning of another noun, according to Richard Upsher Smith, Jr., in A Glossary of Terms in Grammar, Rhetoric, and Prosody for Readers of Greek and Latin: A Vade Mecum.
There are five declensions in Latin. The genitive ending is used in the dictionary because each of the five declensions has its own genitive form. The five genitive terminations are:
An example from each of the 5 declensions:
- puellae - the girl's (puella, -ae, f.)
- servī - the slave's (servus, -ī, m.)
- principis - the chief's (princeps, -ipis, m.)
- cornūs - the horn's (cornū, -ūs, n.)
- dieī - the day's (dies, -eī, m.)