If I am describing a person's personality and physical features, should I use "ser" or "estar"? What about if I am talking about their feelings?
Use "ser" for personality and physical features. You use "ser" for things that are permanent, and a person's personality and physical features are (more or less) permanent. You use "estar" for feelings. Always remember the rhyme "For how you feel or where you are, always use the verb estar."
Why are there so many indigenous people in South America who have kept their traditions and languages? How has this happened in other parts of the world?
The indigenous cultures of South America had less contact overall with European conquerors because of several physical features, particularly the Andes Mountains and the Amazon Rainforest. Since they were more isolated than people on the coasts (the mountains and rainforests being difficult to go through), they were able to keep their traditions and languages (example: Quechua) for longer. This has happened in areas of Asia because of the Himalayan Mountains and the Gobi Desert and the Pacific Islands because they are very spread out.
How did nationalism contribute to the start of World War I?
Nationalism - extreme pride in one's nation - was HUGE in southeast Europe (AKA the Slavic Nations) during the early 1900's. This led to people not liking the governments of the time and thus the assassination of Archduke Franz Ferdinand. The fallout of his assassination caused alliances to kick in and various countries went to war against one another.