There are a few miscellaneous rules associated with the i-stem class that I want to make available, based on crumbs and morsels from A&Gs remaining material.
1. The nominative plural ending -īs (for masc/fem nouns) is possible, but rare, and much rarer than the accusative plural -īs for the same set of nouns.
2. Remember how trusty that plural genitive -ium is supposed to be? It’s supposed to be there for any i-stem noun, whether pure or mixed; masculine, feminine or neuter. Well, not quite…
The following nouns always display -um in place of –ium.
- canis, iuvenis, abāgēs, mare, volūcris, sēdēs, vātēs
The following nouns sometimes or rarely display -um in place of -ium.
- apis, caedēs, clādēs, mēnsis, struēs, subolēs, and the patrials (for more on the patrials, see this post.)
You’ll notice that many of these are mixed i-stems, and their unfaithful genitive plural is one of the features that makes them mixed.
The Essential AG: 77-8