I'm sure Trump knows how to rig the tax code so he pays even less than nothing. He may not be smart himself, but he has enough money to hire smart tax lawyers so he pays no incomes tax.
What other shenanigans and deals he's hiding by not releasing his tax forms we'll never know till after the elections - and even then, only afer Hilary wins.
The tax system is rigged all right - in favor of billionaires like Trump. And he know how to rig it even more - that's why he's trying to buy his way into the Oval Office.