Complex sentence: Charles was then released, leaving three of his sons and sixty Provençal nobles as hostages, promising to pay 30,000 marks and to return a prisoner if the conditions were not fulfilled within three years.
Split into simpler sentences: Charles was then released, leaving three of his sons and sixty Provençal nobles as hostages. He promised to pay 30,000 marks and to return as a prisoner if the conditions were not fulfilled within three years.