Complex sentence: Vytautas was one of the most famous rulers of the Grand Duchy of Lithuania, serving as the Grand Duke from 1401--1430, and as the Prince of Hrodna (1370--1382) and the Prince of Lutsk (1387--1389).
Split into simpler sentences: Vytautas was one of the most famous rulers of the Grand Duchy of Lithuania. He was the Grand Duke from 1401--1430, also the Prince of Hrodna (1370--1382) and the Prince of Lutsk (1387--1389).