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,
-al, 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,
navis).
Allen & Greenough §65-78 discusses I-stems very thoroughly.
flebile nescio quid queritur lyra, flebile lingua murmurat exanimis, respondent flebile ripae