When you prepare your 2020 tax return only include for Box 1 the expenses that applied to 2020. Then when you prepare 2021 taxes, you will enter the amount paid for the 2021 expenses.
The general rule is that you can deduct (or use as the basis for a credit) on your income tax return those qualified educational expenses that you actually paid in that tax year, for attendance at an eligible educational institution during that tax year.
However, there is an exception (to allow for the way colleges actually register students for classes and issue bills) that you can deduct amounts paid in a tax year, for an academic period beginning in the first three months of the next tax year.
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