We form the plural of nouns that end with a consonant + “y” by changing the “y” to “i” and adding “-es:” For example, <em>city - cities, lady - ladies, activity - activities</em>. The only exception is when creating the plural of proper nouns ending in consonant + “y” such as <em>Kennedy, </em>whose plural is<em> Kennedys</em>.
On the other hand, when a noun ends with a vowel + “y”, we form its plural adding an “s” only. For example, <em>monkey - monkeys, boy - boys, valley - valleys.</em>