Complex sentence: After an unsuccessful year at the plate the Giants tried to convert Nathan into a pitcher, but he refused and left to return to Stony Brook for a year, graduating with a degree in business management.
Split into simpler sentences: After an unsuccessful year at the plate, the Giants tried to convert Nathan into a pitcher, but he refused to do that and left the game. He went back to Stony Brook for a year, graduating with a degree in business management.