suffixes

empty-empties
monkey-monkeys
security-securities
holiday-holidays
pity-pities
friendly-i don't know this one
strawberry-strawberries
delay-delays
birthday-birthdays
say-says
vocabulary-vocabularies
conspiracy-conspiracies
memory-memories
candy-candys
story-stories
handy-handyness
envy-don't know this one
library-libraries

2 answers

friendly -- friendlier, friendliness

candy -- candies (follows the rule to change the y to i and add -es)

handy -- handiness

envy -- envies (follows the rule to change the y to i and add -es)

The rest are correct. :-)
friendliness, candied or candies, handiness, envied or envies

it may help to use a dictionary if you are unsure of a word. They provide suffixes. :)