Where does the practice of using -a and -i for plural forms of -um and -us, respectively, come from?
- Bacteria vs. bacterium
- Fungi vs. fungus
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These words are loan words from Latin. The plurals associated with words ending in -um or -us are not dictated by practice, but by precise, Latin, rules.
In Latin - which is an inflected language - there are 5 declensions. Nouns are distributed among declensions and follow declension-specific rules.
So, a noun belonging to the second declension and ending in -us (such as lupus), will have lupi as plural, while one belonging to the same declension and ending in -um will have an -a plural (bellum -> bella).
Note that in Latin nouns have a gender, so lupus is male, while bellum is neuter.
A noun belonging to the fourth declension such as spiritus (male) will have spiritus as plural.