Complex sentence: Another driver, David Purley, upon witnessing the crash, pulled over and abandoned his own race in a desperate and valiant attempt to rescue Williamson, but he was unable to put out the fire or turn the car upright.
Split into simpler sentences: Another driver, David Purley, upon witnessing the crash, abandoned his own race and pulled over in a desperate and valiant attempt to rescue Williamson. Purley was unable to put out the fire or turn the car upright.