How is the modern era different from the postclassical era?

The difference between the two eras is that: The Modern Age is the third of the historical periods in which universal history is conventionally divided, between the fifteenth and eighteenth centuries. Chronologically it harbors a period whose beginning can be fixed in the fall of Constantinople (1453) or in the discovery of America (1492), and whose end can be placed in the French Revolution (1789) or in the end of the previous decade, after the independence of the United States (1776). However, the Postclassic period begins around the year 800 and ends in 1521, when the Spaniards took the capital of the Aztec empire. The phenomenon that characterizes the Postclassic is the invasion of Mesoamerica by semi-nomadic peoples that came from the north, from the vast expanse of Aridoamerica. These people settled in Mesoamerica, mixed with ancient assimilating settlers many elements of classical cultures. With time, they would create a new civilization, comparable to the most advanced of the American continent.

c. the dred scott v. sandford case,

d.summerains, the last one