When was the last time you stumbled onto a word that didn’t end in “s” when it became plural? Or a plural that takes on a completely different spelling from the singular? The English language gives us some delightful little treats in the form of irregular plurals. These words with weird plural spellings should tickle your senses.
Hippopotamus / Hippopotami
One hippopotamus, two hippopotami, three hippopotami...
When it comes to the animal kingdom, many species have irregular plural spellings. In Latin, a word ending in "s" turns into an "i" in the plural form. Most English words borrowed from Latin will take the English rules for plurals, but not "hippopotamus." You would be correct calling a group of them either "hippopotami" or "hippopotamuses." But this quirky allowance isn't extended to all Latin-inspired names. Unfortunately, several octopuses are not "octopi."
Moose / Moose
Elsewhere in the animal world, you have names that are the same word in singular and plural form. These are also known as “zero plurals.” One moose, three moose, ten moose. They’re all just "moose." No "meese" or "moosi" to be found.
The same applies to "deer," "sheep," "walrus," "fish," and "salmon," just to name a few. Some nouns turn into zero plurals, depending on the context. "Keep an eye out for bears!" versus "I'm going hunting for bear this weekend."
Datum / Data
Sometimes you may not realize a word is plural unless you have knowledge of Latin. For example, "data" is the plural of "datum" (so you should be writing “data are” and not “is”).
Syllabus / Syllabuses / Syllabi
Sometimes you have a choice when it comes to making a noun plural. If you want to talk about a stack of these passed out on the first day of class, you can use either "syllabi" or "syllabuses." Your choice.
Attorney General / Attorneys General
This one may sound unnatural to your ears, but it is correct. If there’s a convention for people who hold the title "attorney general," it would be "attorneys general," not "attorney generals." It’s also "senators elect," "poets laureate," "mothers-in-law," and "editors-in-chief."
Cul-de-sac / Culs-de-sac
Terms borrowed from other modern languages can break the plural rules as well. “Cul-de-sac” (French for bottom of the bag) is the singular, but if you’re winding around a neighborhood full of them, they’re “culs-de-sac.” But don't fret too much — following the French rules, they're pronounced the same.