In the book Harry Potter and The Chamber of Secrets, Chapter 2, page 14, last paragrapgh appears 2 sentences like this, "They lets Dobby get on with it" and "Sometimes they reminds me to do extra punishments...". My questions is Why is the third-person rules used with REMIND and LET? thnx
Because that's how Dobby speaks! It's meant to be non-standard English, like his referring to himself in the third person. It's how house elves in general speak in the Potter universe, I believe. Kreacher also referred to himself in the third person, though, off the top of my head, I don't remember what he did with third-person plural verbs.
You'll find the same kind of thing in Tolkien's books with the character Gollum, who also refers to himself in the third person (indeed, for some reason, lack of a grasp of pronouns is common across literature and the media for creatures of limited intelligence or limited grasp of the language, or for robots) and also refers to hobbits as "hobbitses". Wikipedia covers Gollum's speech.