Most English nouns form their plural simply by adding -s or -es: cat/cats, bus/buses, box/boxes. However, as Oxford Learner's Dictionaries records, a significant number of high-frequency nouns are irregular — they form their plural in ways that cannot be predicted from the regular rule. These include ancient English vowel-change plurals (man/men, foot/feet, tooth/teeth), -en plurals (child/children, ox/oxen), zero-change plurals (sheep/sheep, deer/deer, fish/fish), and borrowed Latin and Greek forms (criterion/criteria, phenomenon/phenomena, analysis/analyses). Knowing these irregular forms is essential for fluency because they appear constantly in everyday speech and writing. A learner who writes “childs,” “foots,” or “criterions” will immediately be noticed, as these forms mark a gap in core vocabulary knowledge. Grouping irregular plurals by pattern makes them much easier to learn and remember.
Formation / Rules
| Pattern | Singular | Plural |
|---|---|---|
| Vowel change | man, woman, foot, tooth, mouse, goose | men, women, feet, teeth, mice, geese |
| -en ending | child, ox | children, oxen |
| Zero plural (unchanged) | sheep, deer, fish, species, aircraft | sheep, deer, fish, species, aircraft |
| -f/-fe → -ves | leaf, knife, wife, life, half | leaves, knives, wives, lives, halves |
| Latin -um → -a | datum, medium, curriculum | data, media, curricula |
| Latin -us → -i | cactus, nucleus, syllabus | cacti, nuclei, syllabi |
| Greek -is → -es | analysis, basis, crisis, thesis | analyses, bases, crises, theses |
| Greek -on → -a | criterion, phenomenon | criteria, phenomena |
Examples
- Three children were playing in the park. (child → children)
- The farmer kept twenty sheep on the hillside. (zero plural)
- She brushed her teeth twice a day. (tooth → teeth)
- The data show a clear trend. (datum → data, Latin)
- The phenomena observed were unexpected. (phenomenon → phenomena)
- All criteria must be met. (criterion → criteria)
- The autumn leaves fell quickly. (leaf → leaves)
- Two women won gold medals. (woman → women)
- The laboratory has three scanning electron microscopes. (regular compound)
- The analyses were published last month. (analysis → analyses)
Common Mistakes
| Wrong | Correct | Pattern |
|---|---|---|
| childs / childrens | children | -en plural; no additional -s |
| criterions | criteria | Greek -on/-a |
| phenomenons | phenomena | Greek -on/-a |
| deers / sheeps | deer / sheep | Zero plural — no -s added |
| roofs → rooves | roofs | Not all -f nouns change to -ves |
Practise
Grammar Quiz
Choose the correct plural form for irregular nouns.
Cloze Dropdown
Select the correct irregular plural in context.
Complete the Sentence
Write the correct plural form of the noun given.
Practice What You’ve Learned
LexFizz has 30 free interactive exercises — no sign-up needed.
Browse All Grammar Topics →Explore related grammar topics:
Frequently Asked Questions
What is an irregular plural noun?
An irregular plural noun forms its plural in a way other than adding -s or -es. Examples include: child/children, man/men, mouse/mice, tooth/teeth, foot/feet, goose/geese, ox/oxen, and nouns with Latin or Greek origins like criterion/criteria, phenomenon/phenomena.
Which nouns have the same singular and plural form?
Zero-plural nouns have the same form for singular and plural: one sheep/two sheep, one fish/many fish, one deer/several deer, one species/many species, one aircraft/many aircraft. These are mostly animals and some technical terms.
What are the main patterns for irregular plurals?
Key patterns include: vowel change (man/men, foot/feet, tooth/teeth, mouse/mice); -en ending (child/children, ox/oxen); Latin -um/-a (datum/data, medium/media); Greek -on/-a (criterion/criteria, phenomenon/phenomena); Greek -is/-es (analysis/analyses, crisis/crises).
How do I pluralize nouns ending in -f or -fe?
Many nouns ending in -f or -fe change to -ves: leaf/leaves, loaf/loaves, knife/knives, wife/wives, life/lives. However, some just add -s: roof/roofs, chief/chiefs, cliff/cliffs, belief/beliefs. These must be learned individually.
What is the plural of person?
‘People’ is the standard plural of ‘person’ in most contexts: ten people. ‘Persons’ is used in formal, legal, or technical contexts: persons of interest. ‘Peoples’ means ethnic or national groups: the peoples of Europe.
What are Latin and Greek plurals in English?
English borrowed many academic and scientific words from Latin and Greek and sometimes retains their original plural forms: analysis/analyses, basis/bases, crisis/crises (Greek -is/-es); datum/data, bacterium/bacteria (Latin -um/-a); criterion/criteria, phenomenon/phenomena (Greek -on/-a).
Is ‘data’ singular or plural?
Technically, ‘data’ is the plural of ‘datum’ (Latin), so ‘the data show’ is strictly correct. However, ‘data’ is increasingly treated as uncountable singular in everyday English. Academic and scientific writing still usually treats data as plural.
What is the plural of ‘formula’?
‘Formula’ has two accepted plurals: ‘formulas’ (standard English plural, preferred in non-scientific contexts) and ‘formulae’ (Latin plural, used in mathematics and science). Both are correct; choose based on context and style guide.
Do compound nouns have irregular plurals?
Yes. Compound nouns form plurals on their main word: mothers-in-law, passers-by, courts martial, attorneys general. When the compound has no obvious head word, pluralize the last element: forget-me-nots, grown-ups, check-ins.
Are irregular plural errors common in IELTS writing?
Yes. Writing ‘childs’ instead of ‘children’, or confusing criteria/criterion and phenomena/phenomenon are common errors in academic writing tasks. These errors affect Grammatical Range and Accuracy scores. Learning the key irregular plurals by pattern is most efficient.