3rd declension noun gen pl ending in -ium/-um

Latin Learning Latin
1

salvete!
still struggling with my basic latin
this question came to mind when I am trying to complete a Latin sentence, and in the near immediate future I think I will be doing Latin composition.
how do we differentiate 3rd decelnsion noun with genitive plural ending in just -um or in -ium?
The text (Orberg grammatica latina) simply tells that ovis, navis, urbs, et alia multa, end their genitive plural in -ium, while the rest out of this particular group seems to end in -um.
is there any hint that we can use to see which noun belong to which group?
thx in advance

2

The rules to distinguish i-stem third declension nouns from regular third declension nouns. I-stems are what we call the the third declension nouns whose genitive plural is -ium.

  1. Nouns which end in -is or -es and have the same number of syllables in the genitive singular, e.g., hostis, hostis, m.

An exception is canis, canis, m/f, which has a regular third declension genitive plural, canum.

  1. Nouns ending in -ns or -rs in the nominative singular, e.g., mons, montis, m.

  2. Nouns of one syllable in the nominative singular whose base ends in two consonants, nox, noctis, f.

Neuter i-stems are rare, ending -al or -e, e.g, animal, animalis, n. or mare, maris, n. Notice that in addition to the genitive plural -ium, these have ablative singular in -ī.