What is the main function of the federal courts? To interpret laws of the United States. What is the Supreme law of the land? The Constitution of the United States. Of the many bills proposed to be law, what is a tax or revenue bill? A legislative motion that authorizes the government to spend money. Is the Affordable Care Act a tax? No, according to the president, except and unless it's challenged in the Supreme Court and then, yes, according to the Supreme Court.