Donald Trump has refused to release his tax returns before the November election because of a pending audit. He claims "there's nothing to learn from them." • But the one set of tax returns from the Trump organization that we have been able to see so far suggests there may in fact be a lot to learn from them.The Donald J. Trump Foundation is a charitable foundation that, by law, has made publicly available Form 990s, the tax returns that tax-exempt organizations must file each year.My organization, Citizens for Responsibility and Ethics in Washington (CREW), focuses on reducing the influence of money in politics and spends a lot of time reviewing 990s to evaluate whether nonprofit organizations are inappropriately getting involved in politics. We looked at the Trump Foundation's 990s after learning of a $25,000 contribution the foundation had made to a political organization associated with Florida Attorney General Pam Bondi while her office reportedly was considering joining a lawsuit involving Trump University. Private foundations like the Trump Foundation are not allowed to conduct any political activity, including contributing to political organizations. What is most interesting for the current discussion about the release of tax returns, though, is what the Trump Foundation said — and didn't say — about this political contribution on its tax returns.The Trump Foundation didn't report the contribution on its tax returns. But it doesn't end there. The foundation also claimed on its tax forms that it did not transfer any money to a political organization or engage in any political activity. It did, however, claim that it gave a donation of the same size to a similarly named nonprofit group. The group associated with Attorney General Bondi was called And Justice for All. The foundation's tax return instead listed a $25,000 gift to Justice for All, a Kansas-based nonprofit. The Kansas group has said it never got the money; Bondi's Florida group did.It is not clear whether the Trump Foundation was trying to hide an illegal political contribution or just made an astounding series of errors, and the Trump organization has done little to clear up this point. The foundation's representatives — the same people who help run Mr. Trump's businesses and make decisions about his personal funds — have acknowledged that they made a number of errors.They asserted that a clerk in the organization thought that the payment was meant to be made to still another group, a Utah nonprofit called And Justice for All, and accordingly issued the payment from the foundation rather than from another account. Of course, the money never went to the Utah group, and that explanation doesn't clarify why the tax returns then listed the Kansas group, which according to the Trump organization was the result of yet another separate mistake by accountants.Taking all this into account, CREW filed a complaint with the Internal Revenue Service against the Trump Foundation for possible false statements on tax reruns and potential violation of its tax-exempt status.Whether this incident represents misconduct or vast incompetence, it is certainly relevant information for the public to evaluate. It is exactly the kind of issue that tax returns can illuminate.All major party nominees have made their tax returns public since 1980, and CREW has called on candidates to do so in the past when they've been reluctant. We know from experience how much you can learn from a tax return. Tax returns can tell you about a candidate's income, associations, business dealings, charitable giving, and tax shelters, as well as about the candidate's truthfulness and competence, among many other things.The one example we have from the Trump organization so far raises some troubling questions. By releasing his personal tax returns, Donald Trump can clarify whether the Trump Foundation's problematic tax filings represent an isolated instance, or are indicative of a broader set of problems. Until then, what we've seen gives us little reason to give him the benefit of the doubt.Noah Bookbinder is the executive director of Citizens for Responsibility and Ethics in Washington. He wrote this exclusively for the Tampa Bay Times.