There are many other less regular ways of forming plurals. While they may seem quirky, they usually stem from older forms of English or from foreign borrowings.
Where a noun ends in a sibilant sound—such as s, sh, x, soft ch—the plural is formed by adding es (also pronounced as z with a neutral vowel sound or short i):
Nouns ending in o preceded by a consonant also form their plurals by adding es:
canto[?] cantos grotto[?] grottos piano pianos portico[?] porticos quarto quartos solo[?] solos
Most nouns ending in f or fe form their plurals by changing the f into a v and adding es:
calf calves half halves
Some just add an s:
proof proofs muff muffs
Some can do either:
Nouns ending in y preceded by a consonant change their ending to ies:
cherry cherries lady ladies
The plural of a few Germanic nouns can also be formed from the singular by adding n or en:
The plural is sometimes formed by simply changing the vowel sound of the singular, in a process called ablaut (these are sometimes called mutated plurals):
Some nouns have singular and plural alike:
Plurals for the names of numbers differ according to how they are used. Such words include dozen, hundred, thousand, million, and so forth. The following examples apply to all of these.
Some nouns have no singular form:
annals[?] billiards cattle clothes measles nuptials[?] thanks tidings victuals / vittles
Note, however, that billiard as a singular is used as a number in some versions of British English for 1015 (others will call this a thousand billion), but when speaking of the table game, only exists as plural.
Neither do some names of things having two parts:
scissors trousers tweezers pants
Note, however, that the fashion industry frequently calls a single pair of pants a pant; this is a back-formation.
Some words in which the modifier follows the noun form the plural inside the word or phrase, particularly legal terms from French:
attorney general attorneys general son-in-law sons-in-law court martial courts martial armful armsful / armfuls (the latter is preferred today) governor-general governors-general Knight Hospitaller Knights Hospitallers
It is common in non-formal speech to pluralise the last word in the usual way, but in edited prose, the forms given are preferred.
Some nouns have no plural form:
goodness idleness wisdom
Note, however, that if Wisdom is used as a title ("Your Wisdom") it is then given a regular plural—Their Wisdoms.
Some nouns have two plurals, one used to refer to a number of things considered individually, the other to refer to a number of things collectively. In some cases, one of the two is nowadays archaic or dialectal.
brother brothers brethren cannon cannons cannon child childer children cow cows kine die dies dice fish fishes fish penny pennies pence sow sows swine pig pigs swine iris iris irises
A final odd case is person. The word people is usually treated as the suppletive plural of person (one person, many people). However, in legal and other formal contexts, the plural of person is persons; furthermore, people can also be a singular noun with its own plural (for example, "We are many persons, from many peoples").
Symbols and abbreviations whose plural would be ambiguous if only an s were added are pluralized by adding 's.
mind your p's and q's
Regular words are never pluralized in this way, nor are abbreviations made from initials without periods, as in PCs and ICBMs.
Because English includes words from so many ancestral languages, as well as many loanwords from Classical Greek and Latin and other modern languages, there are many other forms of plurals. Such nouns often retain their original plurals, at least for some time after they are introduced. In some cases both forms are still vying for attention: for example, for a librarian, the plural of appendix is appendices (following the original language); for physicians, however, the plural of appendix is appendixes. Likewise, an electrician works with antennas and an entomologist deals with antennae. The "correct" form is the one that sounds the better in context.
Correctly formed Latin plurals are the most acceptable, and indeed are often required, in academic and scientific contexts.
formula formulae / formulas alumna alumnae
vertex vertices index indices / indexes
phenomenon phenomena (more below) criterion criteria automaton automata polyhedron polyhedra
addendum[?] addenda memorandum[?] memoranda / memorandums medium media
Note: in Latin, virus has no plural form. Viri and virii are both incorrect.
Though some take s more commonly:
cactus cactuses hippopotamus hippopotamuses octopus octopuses platypus platypuses / platypus rhinoceros rhinoceroses / rhinoceros
Of course, the Latinate i plural is frequently heard for these words, but it is considered an error of pedantic hypercorrectness that is not generally accepted in formal use despite having made its way into some dictionaries. (The Greek plural for words ending in -pus meaning "foot", is podes, but that plural is not used in English.)
beau beaux chateau chateaux
cherub cherubim / cherubs seraph seraphim / seraphs matzoh matzot / matzos
Note: kimonos, following the French model, is now generally accepted in English.
Nouns from languages that have donated few words to English, and that are spoken by relatively few English-speakers, generally form plurals as if they were native English words:
Some words of foreign origin are much better known in the plural, while the singular is hardly ever heard except from the fully precise, and with the plural usually taking a singular verb:
candelabrum[?] candelabra datum data agendum agenda graffito graffiti insigne insignia viscus viscera alga algae opus opera phalanx phalanges
Insignia is sometimes counted as a singular form with insignias as its plural, but this usage comes across as illiterate to many people. The singular form insigne is now very rare and sounds absurdly pedantic, so the safest bet is to use insignia only as a plural and to substitute a close synonym (such as symbol or emblem) in place of the singular.
There is an even worse problem with the word data. Although its use as a mass noun is gaining acceptance, to many people it sounds wrong whether used as a singular or as a plural! "The data is" seems jarringly incorrect but "the data are" seems equally jarringly pedantic. The safe way is to use the word only if it would be correct whether singular or plural (for example, "we checked the data"), and to substitute a synonym in other contexts (for example, "the figures are" or "the information is").
A related phenomenon is the confusion of a foreign plural for its singular form:
phenomenon phenomena criterion criteria symposium symposia
Mouses is sometimes seen for computer pointing devices, although mice is probably more common.
Some words have started out with unusually formed singulars and plurals, but more "normal" singular-plural pairs have resulted. For an example from the vegetable world, pease was the singular and peasen the plural, but over the centuries, first pease became the plural and pea the singular, and finally the plural was altered to peas. Similarly, termites and primates were the three-syllable plurals of termes and primas, respectively, but these singulars were lost, the plurals given two syllables, and now we have termite and termites and primate and primates. Syringe is a back formation from syringes, itself the plural of syrinx, a musical instrument. Finally, phases was once the plural of phasis, but the singular is now phase.
Kudos is a singular Greek word meaning praise, but the same process may be happening to it. At present, kudo is an error, however.
There are several different rules for this.
In discussing peoples whose demonym takes -man or -woman, there are two options: pluralize to -men or -women if referring to individuals, and use the root alone if referring to the whole nation.
Englishman Englishmen the English Frenchwoman Frenchwomen the French Dutchman or Dutch people the Dutch Dutchwoman
This also applies to the Irish and the Welsh. One can say "a Scots(wo)man" or "a Scot", "Scots(wo)men", "Scottish people", or "Scots," and "the Scottish" or "the Scots". (Scotch is a drink.)
Several peoples have names that are simple nouns and can be pluralized:
Dane Danes the Danes (or) the Danish Finn Finns the Finns (or) the Finnish Swede Swedes the Swedes (or) the Swedish Spaniard Spaniards the Spaniards (or) the Spanish (much more common)
The term spaniard is most commonly used to refer to a Spanish-speaking inhabitant of Spain (as opposed to a Spanish speaker in another Spanish-speaking country).
Names of peoples that end in -ese take no plural:
Chinese Chinese the Chinese (or Chinese people)
Neither does Swiss or Quebecois.
Most names for American Aboriginal groups are not pluralized:
Blackfoot Blackfoot Ojibwa Ojibwa Iroquois Iroquois Blood Blood Mi'kmaq Mi'kmaq
Some exceptions include Crees, Mohawks, Hurons, Algonquins, Chippewas, Oneidas, Aztecs. Note also:
Most other peoples of the world are pluralized using the normal English rules.