A plural noun is a modified form of a singular noun. Singular nouns are words denoting one, and only one, thing, place, person, or idea. By modifying the noun into its plural form, which in most languages is done according to certain rules and conventions, a word is formed that denotes more than one of the particular thing indicated by the singular form of the noun.
In English, nouns are grouped into two categories, regular and irregular, when discussing the rules and conventions for converting them to their plural forms. Most nouns are regular and pluralization is very simple, consisting of adding the ending -s to the word. A word that already ends in -s, -x or -ch is usually made into a plural noun by the addition of -es. In some cases, the letters -ses may be used to pluralize a word ending in -s.
Numerous special cases exist in English that do not conform to these basic rules. Many rules for words with specific ending letters exist, such as words ending in -o, -f and -y. Words that end in the letter -y are pluralized by removing the last letter and adding the letters -ies, except in the case of proper nouns. Words ending in -f are generally pluralized by changing the final letter from -f to -v and adding -es. A noun ending in -o is usually transformed into a plural noun by adding -es, but there are numerous exceptions.
For irregular nouns, sometimes called mutable or mutating nouns, the plural form is constructed in some other way, not following standard rules for pluralization. These forms must simply be learned through memorization. Some words with Latin roots follow the conventions of that language. Some abbreviations used as nouns are made into their plural noun forms by the addition of -'s, but this is not always the case, either. Some words have unique methods of pluralization.
In some cases, a word appears to be a plural noun because it ends in -s but is not a plural noun at all. Special plural nouns called collective nouns use a singular noun to denote a group of individuals. This type of plural noun may, in some cases, be pluralized in accordance with traditional rules or conventions, if the individual elements of the group act as one unit. An example is the word flock. More than one flock, each of which is made up of a number of individuals, may be referred to as flocks.