>>815639Some plurals like hooves and hoofs as well as rooves and roofs are optional. Both are correct. Choose whichever you like!
But when it comes to the plural of knife its strictly knives, I'm afraid. You don't get to say knifes unless you mean the verb to knife someone i.e. a person attacks another with a blade.
So "one knife; two knives"
"Jemima knifes her friend in the back"
