Latin Grammar Mini-Series | Lesson 16 - Third-Declension Adjectives

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There are two important rules to remember here: 

(1) Third-declension adjectives are i-stem. That means their ablative singulars will end –i, their genitive plurals will end -ium, and neuter nominative and accusative plurals will end -ia. 

(2) Third-declension adjectives can be one-, two-, or three-termination. 

Just like some Latin nouns, some Latin adjectives are third-declension. That is, they have third-declension endings: *, -is, -i, -em, -e, and so on. But unlike third-declension nouns, third-declension adjectives are all i-stem. They have a genitive plural ending -ium, a neuter nominative and accusative plural ending -ia, and an ablative singular ending in all genders, -i. 

Here’s an example of a third-declension adjective: brevis, breve, meaning “short.” 

        M/F I-Stem               Neuter I-Stem

brevis breves breve brevia

brevis brevium    → brevis       brevium

brevi brevibus brevi brevibus

brevem breves/-is breve brevia

breve brevibus brevi brevibus

Some third declension adjectives have two nominative singular forms, for instance, fortis, forte. Fortis is the masculine and feminine form; forte is the neuter. This is called a two-termination adjective.  

Other third-declension adjectives, especially those which have -r at the end of their base, have three nominative singular forms, such as acer, acris, acre. These are called three-termination adjectives. 

For any adjectives which are two- or three-termination, all you do is drop the –is from the nominative singular feminine form and you have the base. 

Take, for example, the two-termination adjective fortis, forte. Fortis is the nominative singular feminine form; drop the –is and you can see that the base is fort-. 

For a three-termination adjective, do the same. So, for acer, acris, acre: the nominative singular feminine form is acris; drop the –is, and you can see that the base is acr-. 

For one-termination adjectives, as we just noted, the genitive singular is required. So, for instance, the base of potens is potent-, which you get when you drop the –is ending from the genitive singular form. 

Let’s do an exercise where you get to practice matching third-declension adjectives with first/second-declension nouns, or nouns of any declension as a matter of fact.

First, we’ll determine the declension of the noun; 

Secondly, its number, gender, and case; 

Thirdly, what is the correct third-declension ending for that same number, gender, and case, then add that ending on to the base dulc- to create the proper form of dulcis that would agree with that particular noun. 

Let’s start simple: puella. 

What declension is it? First, of course! 

And what is its case, number and gender? Nominative singular feminine. 

So now let’s look on the chart. What’s the nominative singular feminine ending in third declension? It’s -is. So the proper form of dulcis that agrees with puella is dulcis. Dulcis puella, or puella dulcis ─ in either case, “sweet girl.” 

Next word: corpus. 

What declension and gender is it? Be careful! Not all -us forms are second declension masculine nominative singular. In case you missed it, that was a hint. It’s third declension neuter.

So what does that make its case and number? It’s nominative singular, and because it’s neuter, it’s also accusative singular. 

So what is the nominative or accusative singular ending in third-declension? Look on the chart. Hello, it’s right there! Yes! It’s -e. So what would be the proper form of dulcis? That’s right, dulce. Corpus dulce, “sweet body.”