The rear seat pilot reported that he was showing the front seat passenger the “basic operation” of a tailwheel airplane. During the three-point landing, with the passenger on the flight controls, the pilot verbally “talked him through the basic steps” of the landing. The landing was unsuccessful and a go-around was initiated. The pilot then took control, stabilized the airplane, and then decided land on the remaining runway. After touchdown, the airplane “turned sharply” to the right, the airplane departed the runway, and impacted a runway sign. The airplane remained upright, and the pilot was able to taxi the airplane back to his hangar. The airplane sustained substantial damage to the left wing forward and aft lift struts. The pilot reported that there were no preimpact mechanical malfunctions or failures that would have precluded normal operation. The pilot additionally reported that the accident could have been prevented by maintaining “aircraft control at all times.”