A method I've used on more than one dog to teach it to bite softly: When they get a bit too rough, or clamp down too hard, grab their front paw, put it in their mouth, shut their mouth down on it with both hands, until you know they've bitten their paw enough to hurt. Do it quickly, then let go. They might yelp, and that's ok. Just do it once. Sometimes they don't realize how biting down hurts others. Normally, it doesn't take more than once for them to realize that clamping down hurts, but I had this one dog that it took twice before he learned the lesson.