The IRS must prove three basic factors beyond a reasonable doubt for a tax evasion conviction: attempt, willfulness, and that the taxpayer had a tax liability. Willfulness is defined as the voluntary, intentional violation of a known legal duty.
As the Eighth Circuit has clarified, “Willfulness, as construed by our prior decisions in criminal tax cases, requires the Government to prove that  the law imposed a duty on the defendant,  that the defendant knew of this duty, and  that he voluntarily and intentionally violated that duty.” See Cheek v. U.S. (1991) 498 U.S. 192, 201. In other words, for the government to prove a taxpayer committed tax evasion, it must prove that the taxpayer acted voluntarily and intentionally when he or she decided to violate the law, and also knew his or her actions were illegal. See also United States v. George, 420 F.3d 991, 999 (9th Cir. 2005).