Third declension

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In the third declension, there are two subcategories: "consonant stem" and "i-stem". They are almost always identical, except in these cases:

• plural genitive: nouns of the "consonant stem" get an ending "-um", while "i-stem" nouns get an "-ium" ending.
• plural accusative in neuter: nouns of the "consonant stem" get an ending "-a", while "i-stem" nouns get an "-ia" ending.
• singular ablative: all words of the "consonant stem" together with the masculine and feminine nouns of the "i-stem" get an ending "-e", while adjectives and neuter nouns of the "i-stem" get an "-i" ending.

How to differentiate between "consonant stem" and "i-stem"?

Nouns of "i-stem" are the following:

1. nouns ending in -is, -es, and having a genitive form with a number of syllables equal to their nominative form. E. g.: civis, civis mf, or collis, collis m;
2. nouns ending in consonant + s (-rs, -ns, -ps, -bs, -x), but only, and exclusively only, if before their genitive ending -is there are at least two consonants. E. g.: gens, gentis, f ("-nt-" is two consonants before the "-is"), or nox, noctis f ("-ct-" is two consonants before the "-is").
3. neuter nouns ending in -e, -al, -ar. E. g.: mare, maris; n, animal, -alis, n; nectar, nectaris, n.
4. almost all adjectives of the third declension

Nouns of "consonant stem" are all nouns of the third declension not matching the requirements listed above.