Complex sentence: He became a midshipman in 1799 aboard the 16-gun sloop HMS Martin, but left her in May 1800 before she was lost with all hands and next served aboard HMS Renown, flagship of Sir John Borlase Warren.
Split into simpler sentences: He became a midshipman in 1799 aboard the 16-gun sloop HMS Martin, but left her in May 1800 before she was lost with all hands. He next served aboard HMS Renown, flagship of Sir John Borlase Warren.