Complex sentence: Plantin took on an assistant, Jan Moretus (Moerentorf), who read Latin and Greek, could write correspondence in several modern languages, became Plantin's business manager, son-in-law and eventually his successor in the Plantin printing press.
Split into simpler sentences: Plantin took on an assistant, Jan Moretus (Moerentorf), who read Latin and Greek and could write correspondence in several modern languages. He became Plantin's business manager, son-in-law and eventually his successor in the Plantin printing press.