Complex sentence: While still a lad he became successively bishop of Bremen, bishop of Verden and coadjutor of Halberstadt, while at the age of eighteen he was the chief commandant of the fortress of Stade.
Split into simpler sentences: While still a lad, he became successively bishop of Bremen, bishop of Verden, and coadjutor of Halberstadt. At the age of eighteen, he was the chief commandant of the fortress of Stade.