Irregular Plurals

Overview

Most English nouns form their plural by adding -s or -es. However, a significant group of nouns follow no predictable rule — their plurals must be memorised. These are called irregular plurals. Knowing them prevents very common mistakes.

1. Internal Vowel Change (Umlaut Plurals)

These nouns change an internal vowel to form the plural.

Singular	Plural	Note
man	men	a → e
woman	women	o → e (pronunciation: /ˈwɪmɪn/)
foot	feet	oo → ee
tooth	teeth	oo → ee
goose	geese	oo → ee
mouse	mice	ou → i
louse	lice	ou → i

2. Completely Different Form

Singular	Plural	Note
child	children	completely different
person	people	completely different
ox	oxen	archaic -en plural

3. Same Form (Zero Plurals)

These nouns are identical in singular and plural form.

Noun	Singular	Plural
sheep	one sheep	fifty sheep
fish	one fish	many fish
deer	one deer	three deer
species	one species	many species
aircraft	one aircraft	ten aircraft
series	one series	two series

There is one sheep in the field.

There are twenty sheep in the field.

4. Latin and Greek Borrowings

Many academic and scientific words keep their original Latin/Greek plural forms.

Singular	Plural	Origin
criterion	criteria	Greek
phenomenon	phenomena	Greek
curriculum	curricula	Latin
datum	data	Latin
formula	formulae / formulas	Latin
analysis	analyses	Greek
basis	bases	Greek
thesis	theses	Greek
index	indices / indexes	Latin
appendix	appendices / appendixes	Latin

5. Words Ending in -f / -fe

These change to -ves (already covered in regular plurals, but note irregular ones too).

Singular	Plural
leaf	leaves
loaf	loaves
half	halves
wolf	wolves
knife	knives
wife	wives
life	lives
self	selves

Exception: roof → roofs (not rooves), chief → chiefs, cliff → cliffs

6. Compound Nouns

For compound nouns, the main noun takes the plural, not the last word.

Singular	Plural
passer-by	passers-by
mother-in-law	mothers-in-law
commander-in-chief	commanders-in-chief
grown-up	grown-ups (last word, no clear head)

7. Common Mistakes

Mistake	Correction	Explanation
mans / mans	men	Irregular vowel change
womans	women	Irregular vowel change
childs	children	Completely irregular
peoples	people	"People" is already plural
sheeps	sheep	Zero plural
criterions	criteria	Greek plural
the datas show	the data show	"Data" is already plural
phenomenons	phenomena	Greek plural

8. Real-World Examples

The children played with the mice in the science lab.

Two women and three men were elected to the committee.

The research data were collected from five species of fish.

The criteria for selection are strict — all theses must be original.

Summary

Type	Pattern	Example
Vowel change	Internal vowel shifts	man → men, tooth → teeth
Different form	Must memorise	child → children
Zero plural	No change	sheep, fish, deer
Latin -um	-um → -a	datum → data
Greek -on	-on → -a	criterion → criteria
Greek -is	-is → -es	analysis → analyses
-f/-fe	-f → -ves	leaf → leaves
Compound	Main noun changes	mother-in-law → mothers-in-law