Solved
In 2017, the 1098-T form from the University reported payments billed. In 2018, the 1098-T form changed to report payments received in a calendar year. For the 2018 tax year, I paid the January 2018 payment in December 2017. When I filed 2018 taxes, I recorded the 1098-T amount paid as it was recorded on the form (which reflected the amount I paid in August 2018 and December 2018 - the Dec. payment was actually for the January 2019 semester. Therefore, I did not include the Dec. 2017 payment in 2018, but instead included the Dec. 2018 payment, included in the 1098-T (although the Dec. 2017 and Dec. 2018 payments were very similar).
You report tuition in the year that you pay it, even if it's for the spring semester of the following year. That's why Form 1098-T was changed to show the actual amount paid during the year. The amount you paid in December 2018 gets reported on your 2018 tax return. You did that correctly. You cannot add the December 2018 payment to your 2019 1098-T or your 2019 tax return, even though it was for classes that started in January 2019. You paid it in 2018, not in 2019. On your 2019 tax return you report only the amount shown on your 2019 1098-T.
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