This page shows the spelling rules for forming plurals in English.
Forming PluralsTabulated below are the rules for forming the plurals of most nouns:
Words Taken from Foreign LanguagesWords "stolen" from other languages form their plurals in a variety of ways.
stadium > stadia or stadiums
(Through common usage, the plural stadiums is acceptable too.)
datum > data
radius > radii or radiuses
(Through common usage, the plural radiuses is acceptable too.)
agendum > agenda
Quirks with forming plurals
Forming the plurals of abbreviations
Forming the plurals of compound nouns