The flight instructor had planned an instructional flight that included practice takeoffs and landings at a nearby airport followed by practice flight maneuvers. Prior to departing, the flight instructor determined that the airplane’s fuel tanks were about 1/2 full, based on the fuel gauges in the cockpit. About 10 minutes into the flight the flight instructor noticed a fuel imbalance, with the left tank reading below 1/2 tank and the right reading above 1/2 tank, so he turned off the left fuel valve leaving the engine to run on the right tank to re-balance the fuel load. After performing two touch-and-go landings and some practice maneuvers, the flight instructor noted that the right tank had dropped to below 1/4 tank, while the left showed between 1/4 and 1/2 tank. The flight instructor then turned on the left fuel valve and turned off the right fuel valve, again, to re-balance the fuel load. Shortly afterward, the flight instructor noticed that the left fuel gauge read empty and the fuel pressure was 0 just as the engine lost all power. He then turned on both fuel valves and both fuel pumps but was unable to restart the engine over the next 20 to 30 seconds before performing a forced landing in a rough field. The airplane’s left wing was substantially damaged during the forced landing. After the accident, the flight instructor stated that there were no preimpact mechanical malfunctions or failures of the airplane that would have precluded normal operation. He further described that he should not have attempted to operate the fuel tanks independently, but rather selected them both on, consistent with the guidance contained in the airplane’s Pilot Operating Handbook (POH). According to the POH, when the fuel level in one of the fuel tanks was below 1/4, both fuel valves must be placed in the on position. Based on this information, it is likely that the airplane’s engine was starved of fuel when the left fuel tank’s fuel supply had been exhausted, after which the flight instructor was unable to restart the airplane’s engine with the limited time remaining.