Adjectives of the third declension have either one, two or three gendered endings.
- Triple-termination: ācer, ācris, ācre (sharp)
- Twin-termination: levis (m/f), leve (light)
- Single-termination: …these are complicated. I’ll address them in a coming post
Triple-Termination Formation
Triple-Termination Third Declension Adjectives are declined as follows:
Here are additional triple-declensions thirds to practice declining:
- alacer, alacris, alacre: lively, cheerful
- campester, campestris, campestre: flat
- celeber, celebris, celebre: famed, crowded
- equester, equestris, equestre: equestrian
- palūster, palūstris, palūstre: boggy
- pedester, pedestris, pedestre: pedestrian, ordinary
- puter, putris, putre: rotting
- salūber, salūbris, salūbre: healthy
- silvester, silvestris, silvestre: woodland
- terrester, terrestris, terrestre: terrestrial
- volucer, volucris, volucre: aerial
- octōber, octōbris, ocrōbre: of October
- (likewise with all menstrual [monthly] adjectives)
Do note: the triple-termination design was developed relatively late, so you may encounter some or all of these adjectives as twin-termination adjectives, with either the masculine or the feminine representing either the masculine or the feminine in early Latin prose and poetry [e.g. homō alacris or fēmina alacer would be acceptable] (AG, 115 n1)
Also, celer, celeris, celere (swift) is an odd bird.
The Essential AG: 115-115a
Latin for Addicts 