The short answer is: We don’t. How do you know that “He goed” is not grammatically correct? At some point in their infancy, most children will say “He goed”. Then they stop saying it, long before a school teacher tells them that it is not good grammar. Good grammar, like vocabulary, is acquired before it
Every summer for the last ten years something magic has happened in Agen, France. Teachers from around the world have gathered in a friendly little town in southwest France and particpated in what many of them have called a life-changing experience. They come together because they have heard of a different way of teaching languages, a way of creating stories with their students and building a different kind of classroom. They come with open hearts and open minds and they leave with smiles and warm memories and many new friends. That is the magic of Agen.