When I take a mortgage of 300 000 at 3% annual rate, will the 3% always be calculated on the 300,000? Or will it be calculated on the remaining principal? i.e if after a year, I paid off 15,000, the next calculation would be 3% based on 300,000 or 285,000?
Simple answer -interest accrues on the principal balance each month, which will decline by the prior month amount paid to principal.
Longer answer - You should search for a sample amortization table. It will clarify this for you and provide insight as to how principal is paid over time. You will see how much money it will take today to knock off one payment at the end of the loan. And you'll also learn why after 15 years, you are not halfway through the payoff amount. Maybe not even 1/3, depending on the rate.