Complex sentence: The Pennsylvania state government left Philadelphia in 1799 and at the time the United States government left Philadelphia in 1800, the city had become one of the United States'busiest ports and the country's largest city with 67,787 people living in Philadelphia and its contiguous suburbs.
Split into simpler sentences: The Pennsylvania state government left Philadelphia in 1799 and the United States government left in 1800. By this time, the city had become one of the United States'busiest ports and the country's largest city, with 67,787 people living in Philadelphia and its contiguous suburbs.