How do you know when to us "i" for plural on a "us" word like "Octopus"?

Question

Some words that end in "us" become "i" when plural, but some don't. How do I know the difference?

Alumnus = Alumni

Octopus = Octopuses

Campus = Campuses

Answer 1

Great question! English borrows words from Latin and Greek, which creates some inconsistencies in plurals. Here’s a guide:

1. Latin Origins ("us" becomes "i"): If the word comes from Latin, it often changes from "us" to "i" in the plural form. Examples:
2. Alumnus becomes alumni.
3. Fungus becomes fungi.
4. Greek Origins ("us" becomes "uses"): Words from Greek often form plurals by adding "es" instead of changing to "i."
5. Octopus becomes octopuses (not octopi, though you'll hear it used casually).
6. Modern English ("us" becomes "uses"): Many modern English words that end in "us" just add "es" for the plural.
7. Campus becomes campuses.

When in doubt, look up the word’s origin or use a dictionary, as English has exceptions!

Answer 2

The main factor is knowing whether the root word was derived from Latin or not. Generally, words derived from Latin (like "alumnus") will have "i" as the ending. But not always! Campus is derived from Latin and is an exception, because it is commonly used as "campuses" in the plural.

Octopus comes from Greek, so the plural is "octopuses". So does "cactus", making the correct plural "cactuses".

Knowing the difference is pretty much dependent on your knowledge of etymology and root words and their origins.