A mortgage payment is more than principal and interest. Property tax and home insurance are usually collected monthly too (escrow), so the number that actually leaves your account each month — often called PITI — is what you should budget around.
What's in the payment
Principal and interest are fixed for a fixed-rate loan and calculated on the amount you finance (home price minus down payment). Property tax and insurance are added on top and can drift over time as assessments and premiums change.
The amortization chart shows how your balance falls — slowly at first, because early payments are mostly interest, then faster as more goes to principal.
Down payment and term
A larger down payment lowers the financed amount and the monthly payment, and a 20% down payment typically avoids private mortgage insurance. A longer term lowers the monthly payment but raises total interest substantially — compare a 15- vs 30-year term to see the trade-off.