Paying a lump sum toward principal once a year doesn't lower the monthly payment — it shortens the loan instead, since every later payment now works against a smaller balance. Made early in the loan's life, this saves far more interest than the same lump sum paid closer to the end, because it skips years of interest that would otherwise have accrued on that money.

🇵🇹 Many people in Portugal use their subsídio de férias or subsídio de Natal — extra income that isn't already earmarked for daily expenses — specifically for this kind of amortização antecipada, once a year.