Complex sentence: In 1519 he left his position and move to Wittenberg, where he began a close professional relationship with Philip Melanchthon, who had recently accepted a position as professor of Greek at the University of Wittenberg on the recommendation of his great-uncle, Martin Luther.
Split into simpler sentences: In 1519 he left his position and move to Wittenberg, where he began a close professional relationship with Philip Melanchthon. Melanchthon had recently accepted a position as professor of Greek at the University of Wittenberg on the recommendation of his great-uncle, Martin Luther.