The US initially does not want to be a part of the war. They hold off as long as they can until Pearl Harbor was bombed. That was why the US was pushed to join the war.
Here is the answer to the first question and it also partially answers the third question. You can just look up bombing of pearl harbor on the internet to see what happened & check in your textbook for the two specific policy changes that are mentioned. A lot were made by FDR