If you have the FTHB credit from 2008 that was a loan, then it is normally repaid over 15 years. However, if you sell, move out, transfer the home to a spouse in a divorce, convert it to a rental, or for any other reason stop using it as your main home, repayment of the remaining credit is immediately due.
The amount of repayment depends on the circumstances of the transaction. If you convert to a rental, the credit is immediately due in full. If you transfer to a spouse in a divorce, the spouse become responsible for your share of the credit. If you sell, you owe repayment up to the full remaining amount from the proceeds of the sale, depending on how much gain you have. For example, if you owe $3000 and you only have a $1000 capital gain on the sale, you pay back the $1000 and then you are free and clear. If you owe $3000 and you have a $10,000 gain, you pay back the full $3000.
All this is reported on a form 5405 for the year you stopped using the house as your main home. If that was not 2017, you will need to go back and file an amended return for the year you stopped using the home as your main home, and pay off in full (or whatever other amount is owed.) You don't owe any repayment in 2017, but your e-filed return might be rejected by the IRS for not including a payment. In that case, print your return and mail it in. If there are years in between when you made $500 repayments that you didn't actually owe, you can amend for those years to get that money back. (In other words, if you sold the house in 2015 and did not file the form, and paid an extra $500 in 2016, then now you need to amend for 2015 and pay off the credit, plus amend for 2016 to get the $500 back.)
