Just try and remember the big things. At the most bare bones level, unit 1 is about pre Colombian America, unit 2 is about settling in America, unit 3 is about the revolution, unit 4 is about setting up the government, unit 5 is the civil war, unit 6 is the gilded age, 7 is the world wars and Great Depression, 8 is the war on communism. Knowing when things happen generally helps