We begin with a list of American rules for irregular plural formation of nouns:
If a word ends with a silent "e,"
drop the "e" before adding a suffix which begins with a vowel (-ing, -able)
Examples: make--making, makable; time--timing, oblige--obliging.
Do not drop the "e"
when the suffix begins with a consonant:
Examples: state--statement; like--likeness; use--useful; definite--definitely.
When "y" is the last letter in a word and the "y" is preceded by a consonant,
change the "y" to "i" before adding any suffix except those beginning with "i"
Examples: beauty--beautiful; fry--fries; hurry--hurried; lady--ladies
When a word ends with a "y" and is preceded by a vowel,
to form the plural of its noun,
or to form the third person singular (he, she, it) of its verb,
Examples: toy--toys; play--plays; monkey--monkeys