Third declension nouns that have the same number of syllables in the genitive as in the nominative genitively have a -ium
genitive plural. Nouns with stems that end in two or more consonants also have this genitive plural (e.g. arc- [arx, arcis]
, lact [lac, lactis]
, Alp- [Alpes, Alpium]
). These are all partial I-stems.
In addition, neuter nouns that end in -e
, or -ar
almost always have a nominative/accusative plural -ia
, genitive plural -ium
, and ablative singular -i
. With just a few exceptions, all 3rd declension adjectives and participles are I-stems. These are all full I-stems. There are several other full I-stems that cannot be identified thus and must be memorized (e.g. sitis
Allen & Greenough §65-78
discusses I-stems very thoroughly.
flebile nescio quid queritur lyra, flebile lingua murmurat exanimis, respondent flebile ripae