This is from the IRS' instructions on page 2 of Form 8396:
Line 1
Enter the interest you paid during the year on the loan amount (certified indebtedness amount) shown on your MCC.
In most cases, this will be the amount in box 1 on Form 1098, Mortgage Interest Statement, or on a similar statement you received from your mortgage holder.
If the loan amount on your MCC is less than your total mortgage loan, you must allocate the interest to determine the part that relates to the loan covered by the MCC.
