Plural forms of nouns

Nouns ending with a long vowel sound and the sound /θ/ have their plural forms pronounced as ending in /ðz/. For example, the plural of path is pronounced /pɑːðz/ and the plural of mouth is pronounced /maʊðz/.
House is pronounced /haʊs/, but its plural form houses is pronounced /'haʊzɪz/.
Note that, if the `ch' at the end of a noun is pronounced as /k/, you add `s', not `es', to form the plural. For example, the plural of stomach /'stʌmək/ is stomachs.
  • stomach -- stomachs
  • monarch -- monarchs

Nouns with no change in form

Some nouns have the same form for both singular and plural.
...a sheep.
...nine sheep.
Many of these nouns refer to animals or fish.
  • bison
  • cod
  • deer
  • fish
  • goldfish
  • greenfly
  • grouse
  • halibut
  • moose
  • mullet
  • reindeer
  • salmon
  • sheep
  • shellfish
  • trout
  • whitebait
Even when a noun referring to an animal has a plural form ending in `s', it is quite common to use the form without `s' to refer to a group of the animals in the context of hunting.
Zebra are a more difficult prey.
Similarly, when you are referring to a large number of trees or plants growing together, you can use the form without `s'. However, this is used like an uncountable noun, not a plural form.
...the rows of willow and cypress that lined the creek.
The following nouns also have the same form for singular and plural:
  • aircraft
  • crossroads
  • dice
  • gallows
  • grapefruit
  • hovercraft
  • insignia
  • mews
  • offspring
  • series
  • spacecraft
  • species

Nouns ending in `f' or `fe'

There are several nouns ending in `f' or `fe' where you form the plural by substituting `ves' for `f' or `fe'.
  • calf -- calves
  • elf -- elves
  • half -- halves
  • knife -- knives
  • leaf -- leaves
  • life -- lives
  • loaf -- loaves
  • scarf -- scarves
  • sheaf -- sheaves
  • shelf -- shelves
  • thief -- thieves
  • wife -- wives
  • wolf -- wolves
The plural of hoof can be hoofs or hooves.

Nouns ending in `o'

With many nouns ending in `o', you just add `s' to form the plural.
  • photo -- photos
  • radio -- radios
However, the following nouns have plurals ending in `oes':
  • domino
  • echo
  • embargo
  • hero
  • potato
  • tomato
  • veto
The following nouns ending in `o' can have plurals ending in either `os' or `oes':
  • buffalo
  • cargo
  • flamingo
  • fresco
  • ghetto
  • innuendo
  • mango
  • manifesto
  • memento
  • mosquito
  • motto
  • salvo
  • stiletto
  • tornado
  • torpedo
  • volcano

Irregular plurals

A few nouns have special plural forms, as shown below:
  • child -- children
  • foot -- feet
  • goose -- geese
  • louse -- lice
  • man -- men
  • mouse -- mice
  • ox -- oxen
  • tooth -- teeth
  • woman -- women
Note that the first syllable of women /'wɪmɪn/ is pronounced differently from that of woman /'wʊmən/.
Most nouns that refer to people and that end with `man', `woman', or `child' have plural forms ending with `men', `women', or `children'.
  • postman -- postmen
  • Englishwoman -- Englishwomen
  • grandchild -- grandchildren
However, the plural forms of German, human, Norman, and Roman are Germans, humans, Normans, and Romans.

Plurals of compound nouns

Most compound nouns have plurals formed by adding `s' to the end of the last word.
  • down-and-out -- down-and-outs
  • swimming pool -- swimming pools
  • tape recorder -- tape recorders
However, in the case of compound nouns that consist of a noun ending in `er' and an adverb such as on or by and that refer to a person, you add `s' to the first word to form the plural.
  • passer-by -- passers-by
  • hanger-on -- hangers-on
Compound nouns consisting of three or more words have plurals formed by adding `s' to the first word when the first word is a noun identifying the type of person or thing you are talking about.
  • brother-in-law -- brothers-in-law
  • bird of prey -- birds of prey

Plurals of foreign words

There are words in English that are borrowed from other languages, especially Latin, and that still form their plurals according to the rules of those languages. Many of them are technical or formal, and some are also used with a regular `s' or `es' plural ending in non-technical or informal contexts. You may need to check these in a dictionary.
Some nouns ending in `us' have plurals ending in `i'.
  • nucleus -- nuclei
  • radius -- radii
  • stimulus -- stimuli
However, other nouns ending in `us' have different plurals.
  • corpus -- corpora
  • genus -- genera
Nouns ending in `um' often have plurals ending in `a'.
  • aquarium -- aquaria
  • memorandum -- memoranda
Some nouns ending in `a' have plurals formed by adding `e'.
  • larva -- larvae
  • vertebra -- vertebrae
Nouns ending in `is' have plurals in which the `is' is replaced by `es'.
  • analysis -- analyses
  • crisis -- crises
  • hypothesis -- hypotheses
Nouns ending in `ix' or `ex' often have plurals ending in `ices'. Some of these have two plural forms, one formed with `s' and one formed in a different way. Usually the form with `s' is used in less formal English.
  • appendix -- appendices or appendixes
  • index -- indices or indexes
  • matrix -- matrices
  • vortex - vortices
Nouns borrowed from Greek that end in `on' have plurals in which the `on' is replaced by `a'.
  • criterion -- criteria
  • phenomenon -- phenomena
The following words borrowed from French have the same written form for the plural as for the singular. The `s' at the end is not pronounced for the singular but is pronounced /z/ for the plural.
  • bourgeois
  • chassis
  • corps
  • patois
  • précis
  • rendezvous
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