Understanding Latin's Third Declension Cases and Endings

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.

According to William Harris:

"The 3rd Declension is in a sense a catch-all for various stem-types, and can be very confusing."

According to James Ross' 18th-century Latin grammar, the nominative singular of a third declension noun may end in:

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

Also, he describes the endings used by different genders:

Nouns can be masculine (especially with endings in -er, -or, -os, -n, or -o); feminine (especially -do, and -go endings); or neuter (especially nouns ending in -c, -a, -l, -e, -t, -ar, -men, -ur, or -us) in gender.

Basic 3rd Declension Types

Third Declension nouns may have a consonantal or i-stem.

Consonantal

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 stems endings:

Singular (the Second Form Is for the Neuter)

• NOM. -/-
• GEN. -is/-is
• DAT. -i/-i
• ACC. -em/-
• ABL. -e/-e

Plural

• NOM. -es/-a
• GEN. -um/-um
• DAT. -ibus/-ibus
• ACC. -es/-a
• ABL. -ibus/-ibus

Using rex, regis, m. (king), here is the paradigm:

Singular

• NOM. rex
• GEN. regis
• DAT. regi
• ACC. regem
• ABL. rege
• LOC. regi or rege
• VOC. rex

Plural

• NOM. reges
• GEN. regum
• DAT. regibus
• ACC. reges
• ABL. regibus
• LOC. regibus
• VOC. reges

Using opus, operis n. (work), here is the paradigm:

Singular

• NOM. opus
• GEN. operis
• DAT. operi
• ACC. opus
• ABL. opere
• LOC. operi or opere
• VOC. opus

Plural

• NOM. opera
• GEN. operum
• DAT. operibus
• ACC. opera
• ABL. operibus
• LOC. operibus
• VOC. opera

I-Stems

Some 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

Sample 3rd Declensions Nouns Declined

• Pugillares
• Os