Complex sentence: In February 1442, King John married his second wife, the fourteen-year old Byzantine princess Helena Palaiologina; his first wife Amadea of Montferrat having died childless in 1440.
Split into simpler sentences: In February 1442, King John married his second wife, the fourteen-year old Byzantine princess Helena Palaiologina. His first wife Amadea of Montferrat had died childless in 1440.