Nouns ending in -y:
* baby -> babies
* story -> stories
* fly -> flies
* city -> cities
* cherry -> cherries
Nouns ending in -f or -fe:
* knife -> knives
* wife -> wives
* wolf -> wolves
* life -> lives
* leaf -> leaves
Nouns ending in -us:
* cactus -> cacti
* focus -> foci
* radius -> radii
* octopus -> octopodes or octopuses (both are acceptable)
* stimulus -> stimuli
Nouns ending in -is:
* analysis -> analyses
* basis -> bases
* crisis -> crises
* thesis -> theses
* ellipsis -> ellipses
Other irregular plurals:
* child -> children
* man -> men
* woman -> women
* tooth -> teeth
* foot -> feet
* mouse -> mice
* goose -> geese
Note: Some nouns have both regular and irregular plural forms. For example, "octopus" can be "octopuses" or "octopodes".