plurals

English typically forms a plural by adding an 's'. But not all languages are the same, and English quite happily borrows / is descended from many. This leads to several irregular plurals, including

• Adding 'en'
  • One ox, many oxen. But, one box, many boxes. As a joke, one Vax (computer), many Vaxen
  • One child, many children (with baroque etymology)
• changing the internal vowel
  • One mouse, many mice; one louse, many lice. But, one house, many houses.
  • One foot, many feet. But, one boot, many boots.
  • One goose, many geese. (And some have suggested that the plural of mongoose should be multigoose!)
  • One (wo)man, many (wo)men. As a joke in science fiction fandom: one fan, many fen.
  • One sheep, many sheep.
• from the Latin
  • One formula, many formulae
  • One radius, many radii
  • One appendix, many appendices; one matrix, many matrices
  • One phenomenon, many phenomena
  • One medium, many media; one millennium, many millennia
• from the Greek
  • One octopus, many octopodes (OED, if not Webster)

• Some languages have three forms: singular, exactly two plural, more than two plural. For example, One Inuk, two Inuuk, three or more Inuit. This distinction occurs only rarely in English:
  • "between" two things versus "among" several things
  • "each other" for two, "one another" for more