OK Mr. and Ms. know-it-all's. A friend gave me a Sears Craftsman Lawn tractor which has NOT been running for years. When I try to start it, the motor makes about a half turn. When I leave the spark plug OUT, the motor will spin like it is supposed to. Put the plug back in and the motor will not turn.

Hi,
The symptoms youdescribe are usually attributable to valve timing out of adjustment. If it is a Briggs OHV, then the YouTube video may be instructional. Setting is 0.004-0.006" and the video is here:
YouTube - Lawn Mower Repair Valve Adjustment (http://www.youtube.com/watch?v=NjzX-PUjKmo)
Peace,
Clarke