Irregular Plurals: Patterns, Variants, and Examples

Reviewed by the Grammarlyzer Editorial Team · Our Review Standards · Published January 22, 2026 · Updated July 16, 2026

Learn the common forms, then check the noun and meaning when variants compete.

Quick Answer

Irregular plurals do not form solely by applying the usual final -s or -es pattern.

Vowel change: man → men, woman → women, tooth → teeth

-en ending: child → children, ox → oxen

Same form: sheep → sheep, deer → deer; Merriam-Webster records both fish and fishes as plurals.

Borrowed forms: criterion → criteria, phenomenon → phenomena; some borrowed nouns also have an English -s plural.

Memory strategy: Learn high-frequency pairs in sentences, not as a rule that you can apply to every similar ending.

🔑 Key Takeaway

Group familiar patterns, but verify an unfamiliar noun in a current dictionary because meaning and field can change the accepted plural.

The Main Irregular-Plural Patterns

These groups help organize the forms; they do not make every plural predictable. Similar-looking nouns can behave differently, and some nouns allow more than one plural.

Pattern How it forms Examples
Vowel change internal vowel shifts man→men, foot→feet, tooth→teeth, mouse→mice
-en ending adds a nonstandard ending child→children, ox→oxen
No change singular = plural sheep, deer, series, species; fish is also recorded alongside fishes
-f → -ves some f/fe nouns change to -ves leaf→leaves, knife→knives, life→lives
Latin / Greek may retain a borrowed plural or add an English ending criterion→criteria; cactus→cacti or cactuses

Common Mistakes

❌ Incorrect:

The selection criterias are strict.

✓ Correct:

The selection criteria are strict.

Criteria is typically plural; criterion is the clearest singular. Dictionaries document additional variation, but criterias is not the standard plural.
❌ Incorrect:

There were three childs in the room.

✓ Correct:

There were three children in the room.

The plural is children, not childs. Do not add another -s to the already plural form.
❌ Incorrect:

The dentist checked my tooths.

✓ Correct:

The dentist checked my teeth.

Tooth uses the vowel-change plural teeth, like foot → feet and goose → geese.
Recorded plural:

We caught several fishes in the lake.

Recorded plural:

We caught several fish in the lake.

Merriam-Webster records both forms as plurals but does not assign them a frequency or species-based division. When the distinction matters, use a context-specific authority or publication guide and apply its choice consistently.

🎯 Test Your Knowledge

1. The plural of "child" is ___.

2. Two important ___ guided the decision.

3. The table is six ___ long.

4. The chef sharpened all the ___.

5. The farmer counted forty ___.

See It Live: Irregular Plurals, Checked

In the exact July 16, 2026 test, the checker marked both childrens and tooths as spelling issues. It offered children among the first word's suggestions but did not offer teeth for the second.

The correct version is: The children lost two of their baby teeth. "Children" is already plural (no extra -s), and "tooth" becomes "teeth" by vowel change.

The revision “The children lost two of their baby teeth.” returned no issues. This exact test shows partial spelling assistance; the dictionary and meaning still supply the correct irregular form.

Frequently Asked Questions

What makes a plural irregular?

An irregular plural does not use the usual final -s or -es pattern alone. It may change an internal vowel, add an older ending, stay unchanged, or retain a borrowed-language form.

What is the plural of "person"?

People is the usual plural of person. Persons remains standard in some law-related contexts, while peoples refers to distinct cultural or ethnic groups.

Is it "fish" or "fishes"?

Merriam-Webster records both fish and fishes as plurals. Its entry does not assign the two forms a frequency or species-based division, so use a context-specific authority or publication guide when the distinction matters.

Is the plural of "mouse" always "mice"?

For the animal, use mice. For a computer mouse, Merriam-Webster records both mice and mouses, so follow the terminology used by your product or publication.

Is "criteria" singular or plural?

Criteria is typically plural and criterion is its singular form. Merriam-Webster also records criterions and notes evidence for singular criteria, but one criterion and several criteria remain the clearest edited pair.

Source-Backed Boundaries

Cambridge's plural-spelling guide establishes the regular -s/-es baseline, notes exceptions, and shows that only some -f/-fe nouns change to -ves.

Merriam-Webster's people entry calls people the usual plural of person and limits much current persons use to law-related contexts. Its fish and mouse entries record plural variants rather than one universal form.

The criterion entry lists criteria and criterions, while noting that criteria is typically plural. The cactus entry likewise records both cacti and cactuses.

A Dictionary-First Editing Workflow

  1. Start with the familiar singular–plural pair: one child, two children.
  2. Check the noun's exact sense. An animal mouse and a computer mouse do not have identical plural options.
  3. If more than one plural is recorded, follow the terminology expected by the field, product, or publication.
  4. Recheck agreement after choosing the noun: one criterion is; several criteria are.

For more practice, see Subject-Verb Agreement and Fewer vs Less.

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Check Your Writing Now

The checker flagged both nonstandard spellings in our exact seed, but it did not suggest teeth. Use its result as one pass, then verify the noun in a dictionary.

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