Some nouns do not follow the general rule of adding
-(e)s to form the plural. Instead they have an irregular plural ending. Here are two important subgroups:

the vowel sound changes: foot-feet, goose-geese, louse-lice, man- men, mouse-mice, tooth-teeth, woman- women. Note also: child-children
A final –f changes to –ves: Calf-calves, half-halves, knife-knives, leaf-leaves, life-lives, shelf-shelves, thief-thieves, wife-wives, wolf-wolves

2. English has a number of nouns with a special meaning associated only with their plural form:

arms (‘våpen’), clothes (‘klær’), customs (‘toll’), damages(‘erstatning’), drugs (‘narkotika’), goods (‘varer’), guts (‘mot’), manners (‘oppførsel’), regards (‘hilsen’), stairs ('innvendig trapp’), surroundings (‘omgivelser’), wages (‘lønn’)

Many of them can be used as regular countable nouns with the same meaning in the singular and plural, but then they have a different meaning; a manner (‘måte’), a custom (‘en vane/skikk’), a drug (‘en medisin’) a cloth (‘et tøystykke/en duk’)

3. Foreign nouns

English has adopted a number of nouns of foreign origin. Their plural forms are irregular and feel foreign. Some have both a foreign plural and a regular plural:

Bacillus (‘basill’) - bacilli, radius – radiuses/radii, formula (‘formel’) – formulae/formulas, datum – data, medium – media, analysis – analyses, axis (‘akse’) - axes, crisis – crises, thesis (‘vitenskapelig oppgave’)- theses, criterion (‘kriterium’) – criteria, phenomenon - phenomena

Introduction