A "forget-me-not" is actually a flower and the word is a noun. So in this context "forget-me-nots" is the plural meaning more than 1 "forget-me-not". Some corrections on your question: "Why is it "nots" in this sentence rather than "not"?" "Is my grammar correct in my question? If I have made any mistakes please point it out for me. Thanks"

I'm assuming your talking about the Frank Sinatra Song "You Make Me Feel So Young". In this song he is talking about a type of flower that is called a Forget-Me-Not. So he uses plural "nots" to say he is picking up a lot of those flowers! I would rephrase this question to: Why does he use "nots" rather than "not" in this phrase?