The private pilot told FAA personnel he was on an instructional flight with a certified flight instructor from a 2600 x 80 foot grass airstrip, which was wet with dew. At approximately 40 knots on initial takeoff roll the flight instructor pulled the left throttle to idle to simulate the loss of an engine on takeoff. The pilot aborted the takeoff, pulling the right throttle back and applying wheel brakes. The airplane slid off the side of the runway into a field and hit a ditch. Examination of the airplane revealed the left wing had pulled forward, separating the trailing edge from the fuselage approximately 1 1/2 inch and buckling the fuselage at the leading edge.