Principal and interest repayments are set so the loan is fully repaid by the end of the term. Early on, most of each repayment is interest; later, most goes to the balance.
Extra repayments go straight off the balance, which reduces the interest charged every period after that. Even small amounts early in a loan save a lot.
Interest-only repayments cover just the interest. The balance stays the same, and repayments jump when the interest-only period ends.
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