A Brief History of Plural Word...s

In order to change a singular noun to its plural form in English, you usually add "s": the plural of pen is pens. The plural of dog is dogs. These are regular plurals. There are many nouns which don't follow this rule. For example, the plural of fish is fish. The plural of man is men. These are irregular plurals.

 

See also: http://en.wikipedia.org/wiki/English_plurals