Complex sentence: The Late Middle Ages was marked by difficulties and calamities including famine, plague, and war, which significantly diminished the population of Europe; between 1347 and 1350, the Black Death killed about a third of Europeans.
Split into simpler sentences: The Late Middle Ages was marked by difficulties and calamities including famine, plague, and war, which significantly diminished the population of Europe. Between 1347 and 1350, the Black Death killed about a third of Europeans.