By changing –y into –ies if a noun ends in a consonant before the -y.

Singular  Plural    Singular  Plural 
army armies   enemy enemies
baby babies   fairy fairies
beauty beauties   family families
berry berries   lady ladies
cherry cherries   library libraries
city cities   nappy nappies
country countries   sky skies
cry cries   spy spies
curry curries   story stories
diary diaries   strawberry strawberries
discovery discoveries   territory territories
duty duties   theory theories