Complex sentence: His foster parents were rewarded for their care of the king's son and were content, but the prince had learned the fate of the girl for whom he had been exchanged and was distraught that he would become king while she suffered.
Split into simpler sentences: His foster parents were rewarded for their care of the king's son and were content. The prince however had learned of the fate of the girl that been exchanged and was distraught that he would become king while she suffered.