3rd Declension I-Stem Rules

I.Masculine and feminine nouns (-ium for genitive plural):

a.) "Parisyllabic": If the nominative ends in -is or -es, and the nominative and genitive have the same number of syllables, then the noun is an i-stem. civis, civis, m/f

b.) If the nominative ends in -s or -x, and the base ends in two or more consonants, the noun is an i-stem. urbs, urbis, f.

II.Neuter nouns (-i in ablative singular, -ia in nominative/accusative plural, -ium in genitive singular):

If the nominative ends in -ar, -al, or -re, the noun is an i-stem. mare, maris, n.