Task: Sentence Expansion
Input (English): The Polish – Lithuanian Commonwealth ( or Union , after 1791 the Commonwealth of Poland ) was a state of Poland and Lithuania ruled by a common monarch .
Output (English): The Polish – Lithuanian Commonwealth – formally , the Crown of the Kingdom of Poland and the Grand Duchy of Lithuania and , after 1791 , the Commonwealth of Poland – was a dual state , a bi-confederation of Poland and Lithuania ruled by a common monarch , who was both King of Poland and Grand Duke of Lithuania .