Irregular Plural Nouns worksheet for students to practice converting singular and plural forms of irregular nouns.

Let’s go step by step to find the correct plural forms for each singular noun. These are irregular plurals, which means they don’t just add “-s” or “-es” like regular nouns. We have to remember their special forms.

We’ll go down the list one by one:

1. trout → This is a fish that doesn’t change in plural. So: trout
2. focus → Changes to foci (pronounced “foe-sigh”) — common in science/math.
3. mouse → Becomes mice — you probably know this from computers or animals!
4. quiz → Just add “-zes” because it ends in “z”: quizzes
5. aircraft → Doesn’t change: aircraft (like “one aircraft, two aircraft”)
6. sheaf → A bundle of grain or paper → becomes sheaves
7. tuna → Fish again — usually stays the same: tuna (though sometimes “tunas” is used, but “tuna” is more standard for multiple)
8. nebula → Space cloud → becomes nebulae (or sometimes “nebulas”, but “nebulae” is traditional)
9. codex → Old book → becomes codices (like “index” → “indices”)
10. wolf → Add “-ves”: wolves

Now the right column:

11. genus → Scientific term → becomes genera
12. curriculum → School course plan → becomes curricula (or “curriculums”, but “curricula” is preferred)
13. deer → Animal that doesn’t change: deer
14. datum → One piece of data → becomes data (yes, “data” is plural!)
15. woman → Becomes women (change “o” to “e”)
16. fish → Usually stays fish, unless talking about different species → then “fishes”. But here, likely fish
17. series → Already plural-looking, but it’s the same in singular and plural: series
18. ox → Becomes oxen (old English form)
19. bison → Like deer, doesn’t change: bison
20. octopus → Can be octopuses (most common) or octopi (less accurate, but often used). We’ll use octopuses since it’s more accepted now.

Let me double-check tricky ones:
- “Quiz” → yes, “quizzes” (double z + es)
- “Sheaf” → “sheaves” (like “leaf” → “leaves”)
- “Nebula” → “nebulae” is classic, though “nebulas” is also okay — we’ll stick with “nebulae” as it’s traditionally taught.
- “Codex” → “codices” matches “index” → “indices”
- “Datum” → definitely “data” is plural
- “Octopus” → modern usage favors “octopuses”

All set!

Final Answer:
trout → trout
focus → foci
mouse → mice
quiz → quizzes
aircraft → aircraft
sheaf → sheaves
tuna → tuna
nebula → nebulae
codex → codices
wolf → wolves
genus → genera
curriculum → curricula
deer → deer
datum → data
woman → women
fish → fish
series → series
ox → oxen
bison → bison
octopus → octopuses