Adiectīva II (Tertiae Dēclīnātiōnis)

Adjectives of the third declension are i-stems and fall into three classes according to how many distinct nominative singular forms they show. The classification reflects an old Indo-European pattern: originally each gender had its own form, but Latin's declensional history has merged some of them.

(I) Three-ending adjectives — distinct masculine, feminine, and neuter in the nominative singular: ācer, ācris, ācre "sharp". Few in number; mostly month-names (September, October) and a small set of descriptive adjectives.

(II) Two-ending adjectives — masculine and feminine identical (-is), neuter different (-e): fortis, forte "strong, brave". The largest class — many hundred words. The form fortis is masc.+fem.; forte is neuter.

(III) One-ending adjectives — single nominative singular for all three genders, with the genitive supplied separately: vetus, veteris (cons.-stem); audāx, audācis (i-stem); prūdēns, prūdentis (i-stem, -ns participle-type). All three genders share the nom. sg. form, but inflect differently in some other forms (especially the neuter, which has its own nom./acc.).

The "one-ending" class is the most morphologically diverse internally:

• Most are i-stems (gen. pl. -ium, abl. sg. -ī when adjectival, n. pl. -ia): audāx, fēlīx, ferōx, ingēns, prūdēns, sapiēns.
• A few preserve consonantal inflection (gen. pl. -um, abl. sg. -e, n. pl. -a): vetus, dīves, pauper, prīnceps, particeps, sospes.

The distinction matters for proper Latin reading and writing: fortis is i-stem with all i-stem features; vetus looks similar but inflects differently and resists the i-stem patterns.

About a dozen adjectives follow this 3-ending type, mostly month-names and a few descriptive:

• Month-names: September, October, November, December. (Note: Iānuārius, Februārius, Mārtius, Aprīlis follow other patterns.)
• Descriptive: celer, celeris, celere "swift"; alacer, alacris, alacre "eager"; volucer, volucris, volucre "winged"; ēquester, ēquestris, ēquestre "equestrian, of the cavalry"; palūster, palūstris, palūstre "marshy"; silvester, silvestris, silvestre "sylvan, wooded".

The pattern can be summarized: nominative singular masculine ends in -er, sometimes preserving and sometimes dropping the e; feminine ends in -(e)ris; neuter ends in -(e)re. Apart from these three nominative singular forms, the declension is identical to fortis.

The ablative singular has special significance:

• Used adjectivally (modifying a noun) → -ī: ā fortī mīlite "by a brave soldier".
• Used substantively (as a noun) → -e: ā sapiente "by a wise person".
• In the ablative absolute with a participle → -e: amante "with a lover loving" (rare construction).

This subtle distinction is rigorously maintained in classical Latin and is one of the marks of accurate writing.

As with 1·2-decl. adjectives, 3rd-decl. adjectives readily serve as nouns. The masculine usually denotes a male person; the feminine, a female; the neuter, a thing or quality.

Examples:

• fortis "a brave (man)"; fortēs "the brave (men)".
• animālia "animals" (n. pl. of animālis, animāle).
• parentēs "parents" (m./f. pl. of parēns, parentis "begetter").
• amantēs "lovers" (m./f. pl. of amāns, amantis).
• omnia "all things" (n. pl. of omnis, omne).
• sapiēns "a wise man" (substantival use of the adjective sapiēns, -entis).

When substantivized, the ablative singular is usually in -e rather than -ī: ab amante "by a lover"; ā sapiente "by a wise man". This rule helps distinguish substantival from adjectival use in writing.

Some adjectives have become essentially fixed nouns: parēns (parent), īnfāns (infant), adolēscēns (young man), iuvenis (young man), senex (old man). These typically appear in dictionaries with their own entries even though their adjectival origins remain visible.

Latin's participles serve as adjectives and decline accordingly:

(1) Present-active participles in -ns, -ntis: amāns (loving), regēns (ruling), audiēns (hearing), capiēns (seizing). They decline like prūdēns — i-stem one-ending. Adjectival ablative singular: -ī; substantival: -e.

(2) Future-active participles in -ūrus, -a, -um: amātūrus (about to love), rectūrus (about to rule). They decline like 1·2-decl. adjectives — like bonus.

(3) Perfect-passive participles in -tus / -sus + -a, -um: amātus (loved), rectus (ruled), victus (conquered). Decline like bonus.

(4) Future-passive participles (gerundives) in -ndus, -a, -um: amandus (to be loved), regendus (to be ruled). Decline like bonus.

Each participle, while functioning as an adjective, retains the verb's tense and voice, and can govern objects. Thus amāns Caesarem "loving Caesar" — the participle agrees with its noun in gender/number/case but governs Caesarem as direct object.

The participle is the bridge between the verbal and adjectival systems of Latin, and is treated more fully in Lectiō XVIII.