I am not a teacher but the way I understand it, the simple rule here is, if the noun ends in "Y" and is preceded by a consonant, change "Y" to "I" and add "ES". Example: strawberry becomes strawberries, pony becomes ponies, bounty becomes bounties.
The word "TOY" is not covered in this rule and therefore will form its plural by adding "S" only.
Hope I was able to help you.
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