Complex sentence: As part of Burgoyne's army, he was captured at Saratoga in 1777 and was a part of the Convention Army until he was exchanged for American General Benjamin Lincoln in 1780.
Split into simpler sentences: As part of Burgoyne's army, he was captured at Saratoga in 1777. He was then a part of the Convention Army until he was exchanged for American General Benjamin Lincoln in 1780.