Rule 1
When there is a consonant before the "y," change the "y" to an "i" before adding a suffix EXCEPT for suffixes starting with i (-ing):
beauty =
beautiful
worry =
worried worrying
lady =
ladies
Rule 2
When there is a vowel before the "y", don't change the "y"; just add the suffix:
say =
saying
turkey =
turkeys
enjoy =
enjoyment, enjoys, enjoyed
There are exceptions!