1. Lift: This is the upward force that opposes gravity and allows the plane to stay airborne. Lift is generated by the shape of the wings, which create a difference in air pressure above and below the wing.

2. Thrust: This is the forward force that propels the plane through the air. Thrust is generated by the engines, which push air backwards to create a forward force.

3. Drag: This is the force that opposes the plane's motion through the air. Drag is caused by friction between the plane and the air.

4. Weight: This is the downward force of gravity acting on the plane.

In summary:

* Lift and thrust are the forces that allow the plane to fly.

* Drag and weight are forces that oppose the plane's flight.

The balance of these four forces determines the plane's flight path and altitude.