Third Declension Adjectives
Third-declension adjectives are the workhorses that modify any noun in the language — omnis vir, omnis fēmina, omne bellum all use one set of endings.
Latin packages them in three flavors by how many forms appear in the nominative singular: ācer, ācris, ācre (three terminations), fortis, forte (two), and ingēns or vetus (one form for all genders).
Most are i-stems, so they take -ī in the ablative singular, -ium in the genitive plural, and -ia in the neuter plural.
The trap is that -ī ablative — students who learned consonant-stem nouns expect -e and miss the agreement. Vetus is the lone holdout that really does keep the consonant-stem -e.
Three nominative shapes — but almost all share i-stem endings in the ablative, genitive plural, and neuter plural.
The four signature i-stem slots (-ī, -ium, -ia, -īs) are what mark these adjectives apart from consonant-stem nouns; vetus is the famous exception.
| Case | SG. M. | SG. F. | SG. N. | PL. M./F. | PL. N. |
|---|---|---|---|---|---|
| Nom. | ācer | ācris | ācre | ācrēs | ācria |
| Gen. | ācris | ācris | ācris | ācrium | ācrium |
| Dat. | ācrī | ācrī | ācrī | ācribus | ācribus |
| Acc. | ācrem | ācrem | ācre | ācrīs (-ēs) | ācria |
| Abl. | ācrī | ācrī | ācrī | ācribus | ācribus |
| Case | SG. M./F. | SG. N. | PL. M./F. | PL. N. |
|---|---|---|---|---|
| Nom. | fortis | forte | fortēs | fortia |
| Gen. | fortis | fortis | fortium | fortium |
| Dat. | fortī | fortī | fortibus | fortibus |
| Acc. | fortem | forte | fortīs (-ēs) | fortia |
| Abl. | fortī | fortī | fortibus | fortibus |
| Case | SG. M./F. | SG. N. | PL. M./F. | PL. N. |
|---|---|---|---|---|
| Nom. | ingēns | ingēns | ingentēs | ingentia |
| Gen. | ingentis | ingentis | ingentium | ingentium |
| Dat. | ingentī | ingentī | ingentibus | ingentibus |
| Acc. | ingentem | ingēns | ingentīs (-ēs) | ingentia |
| Abl. | ingentī (-e) | ingentī (-e) | ingentibus | ingentibus |
| Case | SG. M./F. | SG. N. | PL. M./F. | PL. N. |
|---|---|---|---|---|
| Nom. | vetus | vetus | veterēs | vetera |
| Gen. | veteris | veteris | veterum | veterum |
| Dat. | veterī | veterī | veteribus | veteribus |
| Acc. | veterem | vetus | veterēs | vetera |
| Abl. | vetere (-ī) | vetere (-ī) | veteribus | veteribus |
See It In Action
— B. G. i. 1
The opening line of Caesar puts a two-termination omnis on a feminine noun — same form would cover masculine. One adjective, two genders, no rewrite.
— B. G. i. 30
Communī (i-stem adj. abl. -ī) modifies cōnsiliō (2nd decl. noun abl. -ō) — different declensions, same case. Don't expect endings to rhyme.
— Verg. Aen. xi. 231
Ingentī (one-termination adj.) takes the i-stem -ī abl. while lūctū (4th decl. noun) takes -ū. Same case, different endings — agreement is by case, not by spelling.
— Verg. Aen. i. 215
Vetus genitive veteris — the famous exception. Where a normal third-declension adj. would be omnis, vetus shows it's a true consonant stem (and its abl. sg. is vetere, not veterī).
Both nom. sg. and gen. sg. of two-termination adjs. are -is; verb agreement decides.
fortis vir — "the strong man" (nom.); fortis virī — "of the strong man" (gen.)
Single dative/ablative plural form covers all three genders.
ingentibus murīs — "with huge walls" or "to/for huge walls" — context decides case.
Identical for nom. and acc. pl.; subject-vs-object resolved by the verb.
omnia mūtantur — "all things change" (subj.) vs. omnia videt — "he sees all things" (obj.)
Both endings are valid for the i-stem accusative plural; -īs is older, -ēs later — translate the same way.
omnīs partēs = omnēs partēs — "all the parts" (acc.)
Most third-declension adjectives are i-stems and take -ī — but vetus, dīves, pauper, prīnceps, and participles used as nouns take -e.
abl. sg. ends in -ī
ingentī lūctū
in immense grief
abl. sg. ends in -e
vetere amīcō
by an old friend
Tip: Ask: is this vetus, dīves, pauper, prīnceps, or a participle acting like a noun? If yes, expect -e. Otherwise default to -ī.
In ab amantī muliere, what tells you amantī is functioning as an adjective rather than a noun?
Study Tips
- •Memorize fortis, forte first — the two-termination paradigm is the most common pattern and the cleanest entry point to i-stem endings.
- •When you see an ablative singular ending in -ī on an adjective, expect it to modify ANY gender of noun — the i-stem ablative is gender-blind.
- •Drill the four irregulars together — vetus, dīves, pauper, prīnceps — they take consonant-stem -e in the ablative even though they look third-declension.