Latin Declension Rules | Latin Declensions > Third Declension
A good bet for a Latin noun whose nominative singular ends in -a is that it is a feminine noun of the First Declension. Likewise, a noun ending in -us in the nominative singular is likely Second Declension masculine. There are exceptions, but guessing those is a good starting place. It's not so easy when you get the nouns belonging to the Third Declension.
The 3rd Declension is in a sense a catch-all for various stem-types, and can be very confusing" according to William Harris.
The nominative singular of a third declension noun may end in:
according to James Ross' 18th century Latin grammar, who also describes the endings used by different genders:
a (of Greek origin [for more on declining Greek nouns in Latin, see Latin Third Declension Nouns of Greek Origin]), e, o, c (rare), d, l, n, r, s, t (caput and compounds), or x,
Nouns can be masculine (especially with endings in
-er, -or, -os, -n, or
-go endings); or
neuter (especially nouns ending in
-c, -a, -l, -e, -t, -ar, -men, -ur, or
-us) in gender.
2 Basic 3rd Declension Types
Third Declension nouns may have a consonantal or i-stem.
Note: For the consonantal stems, it may take some practice to figure out where to add the endings, although, the dictionary form should make this clear.
The usual genitive ending of third declension nouns is -is. The letter or syllable before it usually remains throughout the cases.
For the masculine and feminine, the nominative replaces the -is ending of the singular with an -es for the plural. (Remember: neuter plural nominatives and accusatives end in -a.) Similarly, the dative plural is formed from the singular with the addition of -bus. Sometimes the root vowel appears to change, as in our second paradigm word below, opus, operis, n.
First, here are the consonantal-stem's endings:
SINGULAR (the second form is for the neuter)
Using rex, regis, m. (king), here is the paradigm:
LOC. regi or rege
Using opus, operis n. (work), here is the paradigm:
LOC. operi or opere
I-StemsSome nouns of the third declension are called i-stem nouns; still others are mixed i-stem. I-stem nouns have a genitive plural ending in -"ium." Their ablative may not end in "-e," but may instead end in "-i." Other cases may also replace the "-e-" with an "-i-," so you might see an accusative singular ending in "-im." A neuter i-stem noun, animal, animalis (animal), looks a little different from other neuter 3rd declension nouns in the plural because of the "i" which makes the nominative and accusative plural of animal: animalia. The word for sea, mare, maris, is another neuter i-stem noun. Hostis, hostis is a generally masculine i-stem noun, but hostis can be feminine. The fact that the nominative and genitive is the same for this masculine or feminine noun indicates that it's an i-stem.
You would decline the name of Caesar thus:
Caesar, Caesaris, Caesari, Caesarem, Caesare.
For another i-stem, auris, see Auris, Aurum, and Aura.
Please note: Latin Declensions