crisis becomes crises and hypothesis becomes hypotheses

appendix becomes appendices and matrix becomes matrices

child becomes children and ox becomes oxen

bacterium becomes bacteria and curriculum becomes curricula

antenna becomes antennae or antennas and formula becomes formulae or formulas

louse becomes lice and mouse becomes mice

knife becomes knives and self becomes selves

echo becomes echoes and tomato becomes tomatoes

deer becomes deer and offspring becomes offspring

foot becomes feet and goose becomes geese

Here are some observations gathered from the Web to explain this chaos. I’ll cover the consistent ones in this post and the next. Then we’ll look at the inconsistent ones.

For most words, add s.
   elephant becomes elephants
   stereo becomes stereos

For words ending with a hissing sound, add es.
   box becomes boxes
   church becomes churches

For words ending in a vowel plus y, add s.
   tray becomes trays
   keys becomes keys

For words ending in a consonant plus y, change y to ie and add s.
   enemy becomes enemies
   baby becomes babies

For words ending in is, change the is to es and add s.
   synopsis becomes synopses
   thesis becomes theses

For words ending in us, change the us to i.
   cactus becomes cacti
   fungus becomes fungi

sister-in-law becomes sisters-in-law

runner-up becomes runners-up

passer-by becomes passers-by

five-year-old become five-year-olds

greenhouse becomes greenhouses

cupful becomes cupfuls

firefly becomes fireflies

post office becomes post offices