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
These words have these plurals because they are loan words from Latin. Words that come from Latin that end in -um usually have plurals in -a, while those that end in -us have plurals in -i. This way of forming plurals is normal in Latin, and learned English preserves the native Latin plurals.
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.
It comes from people who still remember that a word is a loan word and the lending language was inflected. Often people attempting to inflect the way Latin does do a poor job of it, so outside of the most common Latinisms, it would be better style to use ordinary plurals.
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