He diverted payments to him to his charitable foundation and then directed that foundation to use the money on non-charitable expenditures for his own benefit.
The foundation has admitted to this and amended it previous filings.
Trump would also be required to report the payments he diverted as personal income. He has refused to release returns proving that he did so, and his campaign lied repeatedly about whether he diverted the payments in the first place before admitting that he did so and refusing to answer questions about whether he paid income taxes on the diverted funds.
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It doesn't exactly take a brain surgeon to figure out that he likely didn't report that money as income; if he had they would have said so and probably released evidence in the form of the relevant pages of his returns.
Finally, tax law isn't black and white, legal or illegal, just because it's numbers. People can be "aggressive" or cautious on legality. Think of your own taxes and stuff like deductions for various "business expenses." I think it's very likely that one of the many reasons Trump hasn't released his taxes is that they'll show him being uber-aggressive to the point that some tax experts will conclude he crossed the line.