Grammar guide Updated 2026-10-09

Irregular Plurals in English: A Practical Guide

Most nouns become plural with -s or -es, but a small set of very common nouns does something else: it changes the vowel, adds an old ending, or does not change at all. These forms cannot be guessed and are learned word by word.

The regular path, in one table

1. The vowel change

The oldest plurals were marked by a change inside the word, and a few everyday nouns still work that way: no ending to spot, the whole word shifts.

2. The old -en ending

Two words keep a plural ending that has otherwise disappeared from English: child → children and ox → oxen. Children looks like two plural markers stacked, and historically it almost was. Brother once had brethren, but today brothers is normal.

3. No change at all

These nouns look the same in both numbers, so the verb or the number in front carries the meaning: sheep, deer, fish, aircraft, spacecraft, series, species. Watch two details: fish can take -es for different species, as in the fishes of the reef, and two series or two species are still one form. The countable and uncountable nouns guide takes the group further.

4. -f and -fe: when the sound becomes v

Nouns ending in a spoken -f, and the -fe words, change to -ves. The list is short, and the exceptions matter just as much.

The exceptions tend to be short words where the final f follows a long vowel, which is why the v change never reached them.

5. Words ending in -o take two routes

After a consonant, -o takes either -oes or -os, and no sound rule settles it. A few common words take -oes: potato, tomato, hero, echo. Most others, especially modern and shortened words, take -os: photos, pianos, videos, radios, kilos, logos. Some accept both, including volcano, mosquito, halo, zero and veto. Treat -os as the default and memorise the -oes set.

6. Borrowed plurals from Latin and Greek

Words from Latin or Greek often kept their original ending, and several sit in academic writing.

• -is → -es. analysis → analyses, crisis → crises, thesis → theses, diagnosis → diagnoses.
• -um → -a. datum → data, medium → media, bacterium → bacteria, curriculum → curricula.
• -on → -a. phenomenon → phenomena, criterion → criteria.
• -us → -i. cactus → cacti (also cactuses), nucleus → nuclei.
• -ex / -ix → -ices. index → indices (also indexes), appendix → appendices.

Where both forms exist, the Latin one belongs to formal writing and the English one to everyday speech; mixing them inside one sentence is what reads oddly.

7. Compound nouns and quantity

In a hyphenated compound, the plural goes on the main noun: mother-in-law → mothers-in-law, passer-by → passers-by. When man or woman describes a person, both parts change: a man doctor → men doctors, a woman driver → women drivers; but manhole covers stays regular. For quantities, the unit carries the plural while the noun stays singular: three pairs of shoes, two slices of bread. The much, many and a lot of guide covers the counting words that go with them.

Fix the error: 12 sentences

Each sentence contains one wrong plural. Correct it, then check the notes below.

1. There are three childs playing in the yard.
2. My foots hurt after the long walk.
3. The farmer keeps twenty sheeps in the lower field.
4. Two womans were waiting at the door.
5. We bought two loafs of bread for the weekend.
6. All the rooves were replaced after the storm.
7. He brushed his tooths and went to bed.
8. The cat chased two mouses across the kitchen.
9. She printed twenty photoes for the album.
10. Both mother-in-laws came to the party.
11. Three analysises were published last month.
12. Six gooses swam across the pond.

Answers and explanations

1. children. Child keeps the old -en plural.
2. feet. The oo changes to ee, as in tooth → teeth.
3. sheep. The form does not change, so adding -s is the error.
4. women. Both vowels change.
5. loaves. Loaf belongs with knife, leaf, wife and half.
6. roofs. A trap the other way: roof keeps its -f.
7. teeth. The vowel change again; tooths is an over-correction.
8. mice. Mouse changes its vowel; mouses is not standard.
9. photos. A shortened word takes -os.
10. mothers-in-law. The plural goes on the noun, not the compound.
11. analyses. A Greek word in -is becomes -es.
12. geese. The same oo → ee shift.

How to remember them, and how to drill them

Group the words by the change rather than by topic: vowel change, -en, no change, -ves, borrowed. Then drill the groups.

• Sorting cards. Give pairs singular cards and four labelled columns, and time the sort to three minutes.

More of these are on the classroom grammar drills page, and the verb choices are in the subject-verb agreement guide.

Frequently asked questions

What are irregular plurals?

Most English nouns form the plural with -s or -es, but a high-frequency group changes the vowel (foot to feet), adds an old ending (child to children) or stays the same (sheep). These are learned as individual words rather than built by rule.

Is it roofs or rooves?

Roofs. Roof keeps its -f and simply adds -s, along with chief, belief, cliff, chef, safe, proof and handkerchief. The -ves change belongs to knife, leaf, wife, half, life, wolf, shelf, thief and loaf.

Why is childs wrong but dolls correct?

Child keeps an old plural ending most nouns lost, so it becomes children. Dolls takes -s. Both are standard forms; only children is irregular.

Can I say cactuses or only cacti?

Both are accepted. Many Latin and Greek words keep their original plural and also take a normal English one: cacti or cactuses, indexes or indices. Choose one and keep it consistent.

Is data singular or plural?

Data is the plural of datum, but in everyday English it is widely used with a singular verb, as in The data is clear. In formal writing, keep it plural: The data are clear.

Keep reading

• Countable and Uncountable Nouns: How to Tell Them Apart
• Much, Many and a Lot Of: Choosing the Right Quantifier
• Subject-Verb Agreement: The Rules That Actually Matter
• Classroom Grammar Drills That Work in a 40-Minute Lesson