*Note: a third declension noun is "i-stem" if:

• masculine and feminine nouns with nominative singular in -is or -es and which have the same number of syllables in nominative and genitive sing. (parisyllabic)
• hostis, hostis, m (enemy)
• ignis, ignis, m (fire)
• navis, navis, f (ship)
• moles, molis, f (mass)
• masculine and feminine nouns in -er
• imber, imbris, m (rain)
• linter, lintris, f (boat, skiff)
• uter, utris, m (animal skin)
• venter, ventris, m (belly)
• this is prettty much the entire list
• masculine and feminine nouns which end in -s or -x and whose stems end in two consonants
• ars, artis, f (skill)
• arx, arcis, f (citadel)
• dens, dentis, m (tooth)
• nox, noctis, f (night)
• urbs, urbis, f (city)
• neuter nouns in -al, -ar, and -e
• animal, animalis, n (animal)
• exemplar, exemplaris, n (pattern)
• mare, maris, n (sea)
• i-stems differ from regular third declension nouns only in
• the ABLATIVE singular neuter (-î: animalî)
• GENITIVE plural, all genders (hostium, ignium, urbium, marium)
• NOMINATIVE and ACCUSATIVE  NEUTER plural (-ia: animalia)

• After certain prepositions: sine, de, in, sub, e/ex, cum, pro, a/ab
• Sine rosîs puella tristis est.
• Means: tangible
• Puer puellam rosîs delectat.
• Manner, esp. with adjectives: abstract
• Puer puellam veritate delectat.
• Accompaniment: cum + person
• Cum puellâ puer ambulat.
• Time when: the word in the ablative is a unit of time (usually singular)
• nocte puer ambulat.