The student pilot stated that while on final approach for landing, about 20 feet above ground level, and about 15 feet from the approach end of the runway, the airplane began to sink and it touched down hard off the runway surface. Subsequently, the nose wheel dug into the terrain and the airplane nosed over resulting in substantial damage to the vertical stabilizer and rudder. The pilot reported no preimpact mechanical malfunctions or failures with the airplane that would have precluded normal operation.