Complex sentence: He was the leader of the Praetorian Guard during the first ten years of the reign of the Emperor Claudius, until 51, when the Empress Agrippina the Younger removed him from this position.
Split into simpler sentences: He was the leader of the Praetorian Guard during the first ten years of the reign of the Emperor Claudius. In 51, the Empress Agrippina the Younger removed him from this position.