I'm trying to rationalise my way through the deep and dank forest that is the third declension - in particular, how to know when a noun will take -um or -ium in the genitive plural. I've looked at one or two sites on the web and in Allen and Greenough (I like the way the latter says "The i-declension was confusing even to the Romans themselves,") and I'm still a bit confused. I've tried to take what I've seen and distill it down to the simplest form I can (I'm not that far on in my studies, so simple is good as long as it's not actively misleading).
Third declension nouns take -um in the genitive plural, unless they are:
- A. imparisyllables of the following types
- 1. monosyllabic nom sing nouns ending in cons + -s
- ⁃ ars, artis (f)
⁃ nox, noctis (f)
⁃ os, ossis (n)
⁃ urbs, urbis (f)
2. exceptions which can take either -um or -ium such as:- ⁃ cīvitas, cīvitātis (f)
⁃ parens, parentis (m/f)
3. exceptions which only take -ium, such as- ⁃ līs, lītis (f)
- ⁃ ars, artis (f)
- ⁃ puppis, puppis (f)
⁃ sitis, sitis (f)
⁃ secūris, secūris (f)
⁃ turris, turris (f)
⁃ tussis, tussis (f)
⁃ nūbes, nūbis (f)
- ⁃ animal, animālis (n)
⁃ exemplar, exemplāris (n)
⁃ mare, maris (n)
⁃ rēte, rētis (n)
- ⁃ imber, imbris (m)
⁃ linter, lintris (m/f)
- 1. monosyllabic nom sing nouns ending in cons + -s
Many thanks
David