Salvate
Can anyone help me ?
How can I know that a nomen in the 3. declination is in Genetive Plural -um or -ium. And how can I know if it`s Ablative is -e and not -î
The genitive plural of nomen is nominum, the ablative singular is nomine. The different genitive plural and ablative singular endings that you see in the 3rd declension (i.e. ium or um, i or e) are caused by the fact that some 3rd declension nouns are “i stems”. There is a list of rules for determining whether or not a 3rd declension noun is an i stem in Wheelock ch. 14.
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).
[u]Allen & Greenough §65-78[/u] discusses I-stems very thoroughly.