Complex sentence: By the end of Qianlong Emperor's long reign, the Qing Empire was at its zenith, ruled more than one-third of the world's population, and was the largest economy in the world.
Split into simpler sentences: By the end of Qianlong Emperor's long reign, the Qing Empire was at its zenith. China ruled more than one-third of the world's population, and had the largest economy in the world.