Why are words ending in "-um" and "-us" pluralized to end in "-a" and "-i", respectively?

Solution 1:

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.

Solution 2:

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.

Solution 3:

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.