Complex sentence: The population may have grown from half a million to a million by the mid-fourteenth century when the Black Death reached the country in 1349, and then fallen to as low as half a million by the end of the fifteenth century.
Split into simpler sentences: The population may have grown from half a million to a million by the mid-fourteenth century when the Black Death reached the country. It then may have fallen to as low as half a million by the end of the fifteenth century.