What were the three most significant persons, ideas, or events that led to the Renaissance in the south of Europe? Explain how and why the Renaissance shifted to the North.
The most important cause of the Renaissance was the reintroduction of Classical Roman and Greek texts (especially the works of Plato and Aristotle) that had been largely lost to Europe for centuries. These texts had been preserved by scholars in the Middle East, and after the fall of Constantinople were reintroduced into Europe primarily due to increasing trade between Europe and the Middle East. Of course, Southern Europe is closer to the Middle East than Northern Europe, and even more so than it may at first appear, because travel across water is generally faster than travel across land, so the Mediterranean Sea provided direct trade routes from places like Egypt and Morocco to places like Italy and Spain, while trade with Northern European countries like England and Sweden required crossing the continent.
The second major cause of the Renaissance was the invention of the movable-type printing press, for which Johannes Gutenberg is generally credited (he perfected it, but as usual with major inventions, other people had worked on similar inventions before). The printing press allowed texts to be copied endlessly far faster and cheaper than ever before, which allowed books to spread across Europe and created incentives for people to become literate---at last, there were books worth reading!
But ironically one major trigger for the Renaissance appears to have been the Black Death, the infamous devastating plague that killed as much as a third of Europe's population---millions of people even then. The spread of the Black Death began in the Middle East and spread to Southern Europe and then the rest of Europe, following those same trade routes as the Classical Greek texts. The turmoil created by the plague disrupted existing systems of society and government, opening more room for innovation; and the sudden loss in population actually triggered an increase in per capita wealth. With fewer mouths to feed, much of this wealth was used building educational institutions and funding large projects in art and architecture.