UI am a practicing CPA, and therefore my opinion on Trump's taxes should be taken as that of a tax professional, not those of a speculating, supposing neophyte layman.
In 1995, Trump paid no taxes on his personal return. The reason for this is simple: taxes are paid on income. If you have no income, or if you have business loss, you pay no personal income taxes.
If you are self-employed or you are a corporation, you can roll business losses of one year forward or backward to lessen the taxable income of a subsequent year or a prior year. This is called a "net operating loss carryover." It isn't illegal or unethical, it is part of the tax code. Hillary Clinton and spouse have also used this same provision of the tax code to lower their taxes.
My most recent exposure to the net operating loss provision is in a corporate tax return that was on extension until September 15. The corporation had a loss in 2014, which I rolled forward to 2015, thereby lowering the 2015 taxable income.
Say the corporation had a loss of $6,000 in 2014, and a gain of $10,000 in 2015. The taxable income for 2015 would then be $4,000. ($(6,000) + $10,000 = $4,000).
This provision is part of the tax code and it is commonly used. Hillary Clinton's attempt to make political hay out of Trump's use of this provision is dishonest and misleading -- and she knows it.