Starter clutch can stick, it's behind the alternator rotor and consists of three spring loaded rollers. If the springs and plungers get gummed up then can stick in either the engaged or disengaged position.
To inspect you'll need to remove the alternator cover and pull the rotor off the end of the crankshaft. The first is easy, the second not so much, so eliminate other possibilities first. Once the alternator cover is off you can test the starter clutch by moving the starter gear that's behind the rotor. It should roll easily in one direction and not at all in the other. That'll tell you whether you need to pull the rotor.
mystic_1
