What an amortization schedule actually shows
Amortization is the process of paying a debt down to nothing with a series of equal payments. The schedule is the row-by-row record of that process: for each month, what the payment was, how much of it the lender kept as interest, how much came off the balance, and what was left afterwards.
The mechanism is simple enough to state in a sentence. Interest is charged on what you still owe, your payment covers that interest first, and whatever is left reduces the balance. Because the balance is smaller next month, so is the interest, so a slightly larger slice of the same payment goes on the balance. Repeat three hundred and sixty times and you have a mortgage.
What the table makes visible is how lopsided the early years are. The first payment on a long loan is mostly interest; the last is almost entirely balance. Nothing about that is a trick or a penalty. It falls straight out of charging interest on an amount that starts large.
The three moments worth looking for
A three-hundred-row table hides its own answer. The summary above the schedule names the three moments people are actually looking for.
The crossover is the first month where more of your payment reduces the balance than is taken as interest. On a thirty-year loan at an ordinary rate it lands somewhere around year nineteen, which surprises most people the first time they see it.
The halfway point is the first month the balance is under half of what you borrowed. It is not the middle of the term, and on a long loan it is nowhere near it.
The last payment is almost always a few cents smaller than the rest, and the schedule shows exactly what it is.
Paying extra, and why timing dominates
An extra amount added to every instalment goes entirely against the balance, because the interest for that month has already been covered. Removing balance early removes every future month of interest that balance would have generated, which is why an overpayment in year one is worth many times the same amount in year twenty-five.
Put a figure in the extra payment field and the schedule below switches to the shorter loan that results, so the table you read is the one you would actually pay. The summary keeps both: what the contractual loan would have cost, and what the accelerated one costs instead.