In many languages the parts of speech are inflected differently depending on whether they are related to a noun of whose referent there is only one instance (singular), or several (plural). Several languages also have a dual grammatical number that expresses the existence of precisely two instances of the noun, and a collective number that expresses the whole class of the nouns. Other languages (one of which is English) treat dual nouns as simply plural. Some other languages have a trial number for three or a paucal number, expressing few -- but not many -- instances of a noun, which is separate from the singular or plural numbers. Also, some languages have of collective nouns (e.g. "mankind") that are declined either as singular or plural, but semantically express multitude.

In English the following are irregular examples:

  • house (singular), houses (plural)
  • mouse (singular), mice (plural)
  • I (singular), we (plural)

And one regular example:

  • encyclopedia (singular), encyclopedias (plural)

Non-borrowed English irregular nouns come in several forms:

Some voice a final fricative when in plural:

  • knife, knives (f>v)
  • mouth, mouths (T>D)
  • house, houses, (unique plural, s>z)
These plural are distinct in pronunciation from the possessive. There is also a trend in some areas to regularize some of these nouns.

Survivors of the Old English weak masculine declination add -en:

  • ox, oxen
  • auroch, aurochen (archaic)

Other -en adders are irregular due to different reasons:

  • child, children
  • eye, eyen (rare)
  • cow, kine (rare)
  • brother, brethren (or brothers)

Some nouns have no plural, or are identical when plural and singular:

  • moose
  • sheep
  • fish (or fishes)
  • species

Pronouns are irregular precisely because they are so common:
  • I, we
  • you
  • he she it, they

Some nouns are rather transparently irregular because they undergo the process of umlaut:

man, men foot, feet mouse, mice

There are several different kinds depending in the starting and ending vowel, but generally, they converge on /i/.

Most of these nouns are also umlautized in the other Germanic languages.

The (regular) English noun plural marker, -s, has three variants:

  • -/s/ next to a voiceless consonant other than a fricative
  • -/z/ next to a voiced sound other than a fricative, or a vowel
  • -/@z/ or -/Iz/ next to /s/, /z/, /S/, /Z/, /tS/ and /dZ/ (the choice of vowel depending on dialect)

In Slovene more complicated:

  • babarija (old wives tale) (singular), babariji (two old wives tales) (dual), babarije (three old wives tales)
  • hiša (house) (singular), hiši (two houses) (dual), tri hiše (three houses) (plural), šest hiš (six houses) (plural)
  • miš (mouse) (singular), miši (two or three mice) (dual := plural)
  • jaz (I) (singular), midva/midve (we) (dual + [Masculine/Feminine gender), mi/me (we) (plural [Ma/Fe gender])
  • vrata (one door) (singular), dvoje vrat (two doors (dual), tri vrata (three doors (plural), [plural noun with different or same form]
  • babine (afterbirth period) (archaic meaning) (singular), babini (two afterbirth periods) (dual), babine (three afterbirth periods), [plural noun with different or same form]
  • človeštvo (mankind) (singular), človeštvi (two mankind) (dual), človeštva (three mankind), [collective noun with different form]
    • This kind of examples are very often used incorrecty (even in published or electronic dictionaries).

In Hebrew, one can similarly say:

  • sefer (book) (singular), sfarim (books) (plural)
  • yom (day) (singular), yamim (days) (plural), but yomaim (two days) (dual)

In terms of pronunciation, however, the majority of nouns (and adjectives) in French are not actually declined for number. The -s suffix is not actually pronounced unless the next word starts with a vowel (this is called liaison) and thus does not really show anything; the plural article or other word is the real indicator of plurality. However, plurals still exist in French because irregular nouns, such as those that end in -l such as cheval, horse, form plurals in a different way. Cheval is pronounced [S@val], cheveaux is pronounced [S@vo], and this is a real showing of number differences. The same is true for adjectives.

Not only nouns can be declined by number. In many languages, adjectives are declined according to the number of the noun they modify. For example, in French, one may say un arbre vert (a green tree), and des arbres verts ([some] green trees). The word vert (green), in the singular, becomes verts for the plural (unlike English green, which remains green).

In many languages, verbs are conjugated by number as well. Using French as an example again, one says je vois (I see), but nous voyons (we see). The verb voir (to see) in the first person changes from vois in singular, to voyons in plural. In English this occurs in the third person (she runs, they run) but not first or second.

Normally verbs agree with their subject noun in number. But in Ancient Greek and Sanskrit neuter plurals took a singular verb. In English nouns collectively referring to people may take singular verbs, as the committee are meeting; use of this varies by dialect and level of formality.

Other qualifiers may also agree in number. The English article the does not, the demonstratives this, that do, becoming these, those, and the article a, an is omitted or changed to some in the plural. In French and German the definite articles have gender distinctions in the singular but not the plural. In Portuguese the indefinite article um, uma has plural forms uns, umas.

See grammar, mass noun, collective noun.