The certificated flight instructor (CFI) completed a preflight inspection, and noted that each wing fuel tank contained approximately 18 gallons of fuel before departing on the local instructional flight. At the conclusion of the flight, while returning to the departure airport, the engine experienced a total loss of power. The CFI stated that he maneuvered the airplane for a forced landing to a field while conducting the emergency landing checklist; however, he did not change the fuel tank selector position. The airplane touched down in the field and impacted a row of trees, resulting in substantial damage to both wings. Following the accident, approximately 18 gallons of fuel was observed in the left wing tank, and about 8 ounces of fuel were drained from the right wing tank. During postaccident examination by a Federal Aviation Administration inspector, a fuel line was plumbed to the engine. The inspector then started and ran the engine through all power settings with no anomalies observed.