Here's the rule:
* If the letter before the "y" is a consonant, change the "y" to "i" and add "es" to form the plural or possessive.
In this case, the letter before the "y" is "v" (a consonant). Therefore, we change the "y" to "i" and add "es" to form the plural or possessive form of "envy", which is envies.
Here are some other examples of words that follow this rule:
* baby - babies
* city - cities
* story - stories
* fly - flies
* cry - cries