Irregular plural nouns.

Simple (one-stem) nouns:

a) ending in sibilants

-es

box—boxes, voice—voices, glass—glasses, bridge-bridges, watch—watches, bush—bushes, fish—fishes

b) ending in -y

-ys

day—days, journey—journeys, key—keys, boy—boys

-ies

city—cities, country—countries, penny—pennies, fly-flies, army—armies, lady—ladies

c) ending in -o

-os

cuckoo—cuckoos, piano—pianos, photo—photos, solo—solos, zero—zeros, portfolio—portfolios, soprano—sopranos, ratio—ratios, dynamo—dynamos

-oes

hero—heroes, echo—echoes, embargo—embargoes, Negro—Negroes, potato—potatoes, tomato—tomatoes, veto—vetoes

d) ending in -f, -fe, -fs, -fes

cliff—cliffs, belief—beliefs, chief—chiefs, proof-proofs, roof—roofs, safe—safes

-ves

calf—calves, half—halves, knife—knives, leaf—leaves, life—lives, loaf—loaves, self—selves, sheaf—sheaves, shelf—shelves, thief—thieves, wife—wives, wolf-wolves

e) -en

child—children, ox—oxen

f) vowel interchange

man—men, foot—feet, tooth—teeth, goose—geese, louse—lice, mouse—mice, woman—women

g) homonymous forms

sheep—sheep, deer—deer, grouse—grouse, plaice— plaice, salmon—salmon, pike—pike, trout—trout, carp—carp, moose—moose, fish—fish, cod—cod, series—series, species—species, means—means, (gas) works—(gas) works, barracks—barracks, headquarters—headquarters

h) ending in –us -i

stimulus—stimuli, nucleus—nuclei*, syllabus—syllabi*, focus—foci*, bacillus—bacilli, terminus—termini

i) ending in –a -ae

formula—formulae, antenna—antennae*, vertebra-vertebrae*

j) ending in -um -a

curriculum—curricula, memorandum—memoranda⁴, stratum—strata, addendum—addenda, bacterium bacteria, medium—media*, aquarium—aquaria*, symposium—symposia*, datum—data

k) ending in –is -es

crisis—crises, analysis—analyses, axis—axes, diagnosis—diagnoses, ellipsis—ellipses, hypothesis-hypotheses, oasis—oases, parenthesis—parentheses, synopsis—synopses, thesis—theses, basis—bases

1) ending in -ex/-ix -ices

codex—codices, index—indices*, appendix-appendices, matrix—matrices

Compound nouns:

a) plural in the last element

boy-friend—boy-friends, fountain pen—fountain pens, grown-up—grown-ups, assistant director—assistant directors, attorney general—attorney generals, mouthful—mouthfuls, spoonful—spoonfuls, room­mate—room-mates, woman-hater—woman-haters, breakdown—breakdowns, overall—overalls, sit-in—sit-ins, stand-by—stand-bys, take-off—take-offs, forget-me-not—forget-me-nots, merry-go-round—merry-go-rounds, postman—postmen, Englishman-Englishmen, writing table—writing tables

b) plural in the first element

Notary public—notaries public, passer-by—passers-by, mother-in-law—mothers-in-law, man-of-war—men-of-war, coat-of-mail—coats-of-mail, court-martial—courts-martial, commander-in-chief—commanders-in-chief