For most people, spending five years in the U.S. with a green card and studying up on their English and the 100 potential questions regarding U.S. history and government is enough to qualify them for U.S. citizenship. However, the law is thick with exceptions and more detailed rules. For example, some people can apply sooner than others. Others shouldn't apply yet, or at all, due to excessive time spent outside the U.S., commission of a crime, an extramarital affair or other stain on their good moral character, and so on. Get the lowdown here.