It’s funny, sometimes, how we twist plurals from other languages.
Because English has taken words from many other languages, we have many different ways of making plurals. Some are more true to the origins than others. Some morph in funny ways. And some are just confusing.
“Radius” comes from Latin, and its Latin plural is “radii”, a construction that we usually retain — it’s pretty uncommon to see “radiuses”, though American Heritage accepts it. “Virus” looks related, but its plural is not “viri” (and definitely not “virii”, which would be wrong in any case), but “viruses”. I often jokingly use “irii” as the plural of the iris flower. Jokingly.
But we say “octopi” without jest, though it should be a joke: “octopus” comes from Greek, not Latin, and should not take the “-i” plural. Say “octopuses”, please.
The variations on the Latin word for folks who attended one’s school cause no end to confusion. Here’s the skinny: a male graduate is an “alumnus”, a female graduate is an “alumna”, a group of female graduates are “alumnae” (last syllable sounds like “knee”), and a group of male, mixed-sex, or unknown-sex graduates are “alumni” (last syllable is “nye”). Or just say “alum” and “alums”; casual and short, they’re never wrong.
“Index” and “appendix” become “indices” and “appendices”, but not in some style guides, which prefer to avoid “confusing” plurals. “Datum” and “agendum” have pretty much gone away, replaced in singular usage by their plurals. “Addendum” and “erratum” survive, though, along with their correct plurals, “addenda” and “errata”.
I once had an electronic device that had instructions that “this unit require six pieces of size D battery.” I had images of chopping a battery into six chunks with a cleaver, but I learned the reason for the odd translation. In English, we can say “one battery”, but we can’t say “one grass”. We have to give the grass its unit: “one blade of grass”. In Chinese, it’s true of all nouns that when they get a number, they need a unit. So the “pieces” translation is just direct from the Chinese.
I’ve found some amusement in Italian plurals, which we use for foods: zucchini, cannoli, tortellini, and the like. Yes, those are plurals. So don’t order “two cappuccinos”, but try “due cappuccini”, and see if you get what you want. I’ve wondered about asking for a single cannoli. Shouldn’t I say “cannolo”? No, advised an Italian colleague. It’s true that “cannoli” is plural, but the singular isn’t used: Italians would ask for “un pezzo di cannoli”, one piece from the batch.
Maybe with these foods, the point is that you can’t eat just one, so we just stick with the plurals. Only, we do say “zucchinis” sometimes, when ”zucchini” should do.
The British take care of the problem by calling them “courgettes”, from the French.