THE COMMERCIAL, (CFI), PILOT WAS PERFORMING A TRAINING FLIGHT. DURING SINGLE ENGINE TRAINING, THE LEFT PROPELLER BEGAN AN UNCOMMANDED RETURN TO FLAT PITCH. THE CFI TOOK THE CONTROLS, MADE A PRECAUTIONARY LANDING, AND INSPECTED THE PROPELLER. AFTER AN EXTENSIVE RUNUP, THE CFI ELECTED TO ATTEMPT A TKAEOFF FROM A NEARBY DIRT ROAD. DURING INITIAL CLIMB, AT 20 FEET AGL, THE LEFT PROPELLER WENT INTO FLAT PITCH, CAUSED THE PLANE TO YAW TO THE LEFT, THE LEFT WING DROPPED AND THE PLANE LANDED HARD, WITH THE GEAR RETRACTED. THE LANDING GEAR, BOTH WINGS, AND THE GUSELAGE WERE BENT AND TWISTED. THE LEFT PROPELLER DOME SHOWED EVIDENCE OF OIL LEAKAGE OVER A PERIOD OF TIME. THE LEFT ENGINE CONTAINED 3 QUARTS OF OIL, WHICH WAS BELOW THE MINIMUM LEVEL. ONE PROPELLER BLADE ON THE LEFT ENGINE HAD A BROKEN PITCH CHANGE LINK. THE DOME WAS EXTENSIVELY CRACKED AND HAD BEGUN TO DISINTEGRATE.