A loan payment usually contains both interest and principal. Calculate interest on the beginning outstanding balance using the period rate. Subtract interest from the payment to find principal reduction, then subtract principal from the balance. Expensing the full payment overstates cost and leaves the liability wrong. The final payment may differ slightly because each period is rounded to cents.
A zero-interest loan still repays principal. Interest incurred before the next payment may need a separate year-end accrual. Intangible amortization is a different use of the same word: it allocates the cost of a finite-life intangible over its useful life. The exercises provide assumed rates and lives for practice; they are not lending offers or tax rules.