My kids ask me "do you "去y 加ies" in the plural form of monkey?" I said no, but honestly I can’t think of the logic behind it to defend my reason. It’s the way it is, we adults usually don’t bother to know why.
Fortunately with the help of internet, looking for a specific grammar rules is only a click of mouse away. OWL writing lab offers a complete yet concise rules of spelling here.
So the plural form of monkey fits in the 3rd category, which says that if the word ends in a vowel plus-y, add an -s to the word. ex. key-keys, monkey, monkeys