Then, of course, we have a few words that have no plural. Sheep are sheep. You can’t call them sheeps or sheepses. Which reminds me…real fishermen never call what they catch fishes. Fish are fish, never fishes.
Just to make things interesting, we also have about a dozen words that are quite irregular. We can talk about houses and spouses, but we can’t say that mouses have louses. It’s mice have lice. And it’s definitely not “hice” and “spice.”
But how about plural plurals? One has amused my editor for a while—opus. This word comes from the Latin and means “work.” The word “work” comes from the old Germanic weorc and can be pluralized. For example, “Shakespeare’s works.”
But the plural of opus is opera (“works”) and—in English—the plural of opera is operas. So we do have a plural plural.
Fortunately, time and usage have separated opus and opera. An opera is now a singular word. “Beethoven wrote only one opera, while Scarlatti wrote more than fifty operas.”
The word opus does have a plural form: opuses. But this is gradually giving way to “works” (“the works of Haydn”) and the plural word opuses is very rarely, if ever, used.