So why study this stuff if I know it already?
One goal of this class is to make conscious at least some of the grammatical rules English speakers learned unconsciously as infants and share in common as speakers of English. This is part of knowing about ourselves and those with whom we speak; such knowledge is (or should be) part of our intellectual foundation as educated people. Consider an analogous example: as human beings, we all possess livers, and most of us could lead quite happy and productive lives without knowing anything about how our livers work; but the knowledge you gain about how the liver works in a biology class is part of more general knowledge about what it is to be a human being. Knowledge about how language, or a particular language, works is even more central to knowing what it is to be human, because humans are the only species that acquire a full language naturally.