What declension of a noun do I use?

A noun can only belong to one declension. In Latin, there are five declensions. In Latin I you will cover the first three declensions of nouns. The key to knowing which declension a noun belongs is found within the principal parts. The principal parts give key information. Here is how it works:

Look at the 2nd principal part (Genitive singular)

1st declension nouns:
puella, puellae, f. girl
vita, vitae, f. life
pecunia, pecuniae, f. money
auriga, auriagae, m. charioteer

All 1st declension nouns have a genitive singular ending of -ae so the -ae ending indicates a 1st declension noun. Most are feminine but not all.

2nd declension nouns:
servus, servï, m. slave
puer, puerï, m. boy
caelum, caelï, n. sky, heaven
bellum, bellï, n. war

All 2nd declension nouns have a genitive singular ending of -ï. 2nd declension nouns are either masculine or neuter.

3nd declension nouns:
lux, lucis, f. light
pater, patris, m. father
corpus, corporis, n. body

All 3rd declension nouns have a genitive singular ending of -is.
Also, even though there are only two sets of declension endings in 3rd declension nouns, all three genders are used. Masculine and feminine nouns are declined the same in third declension.

When students memorize the principal parts, they should realize there is a purpose. These parts will give them clues to grammar concepts. If a student is working with the noun "slave" and has memorized the p.p.(principal parts), the student will know to put only 2nd declension endings on servus,etc.

Now you the student must memorize the cases of nouns and their uses!