If a noun ends in y sometimes you change the to i and add es but just s how do know which should do?

You're right, forming the plural of nouns ending in "y" can be tricky! Here's the breakdown:

When to Change "y" to "i" and Add "es":

* If the letter before the "y" is a consonant: The "y" changes to "i" and "es" is added.

* Examples:

* baby -> babies

* fly -> flies

* story -> stories

When to Just Add "s":

* If the letter before the "y" is a vowel: The "y" stays the same and only "s" is added.

* Examples:

* boy -> boys

* monkey -> monkeys

* day -> days

Exceptions:

* Proper nouns: Proper nouns ending in "y" generally just add "s" (e.g., Mary -> Marys).

* Words ending in "quy": These words add "s" (e.g., attorney -> attorneys).

Remember: Practice makes perfect! The more you see and use these rules, the more natural they will become.