To enter mortgage interest:
Per IRS Publication 17 Your Federal Income Tax (For Individuals), page 155:
In most cases, you can deduct all of your home mortgage interest. How much you can deduct depends on the date of the mortgage, the amount of the mortgage, and how you use the mortgage proceeds.
Fully deductible interest. If all of your mortgages fit into one or more of the following three categories at all times during the year, you can deduct all of the interest on those mortgages. (If any one mortgage fits into more than one category, add the debt that fits in each category to your other debt in the same category.)
The three categories are as follows:
The dollar limits for the second and third categories apply to the combined mortgages on your main home and second home.
See Part II of Pub. 936 more detailed definitions of grandfathered, home acquisition and home equity debt.
You can use Figure 23-A (on page 157 of IRS Publication 17) to check whether your home mortgage interest is fully deductible.
Note: You must be legally liable for the mortgage in order to deduct the mortgage interest.