Whereas, at the beginning, the mortgage balance is dropping only very slowly because you've got a lot of interest to pay and your mortgage payment is constant.

而在贷款初期,贷款余款减少的则非常缓慢,这是因为还款额是固定的,同时你还要偿付大量的利息

耶鲁公开课 - 金融市场课程节选

Your mortgage payment is constant through time but the fraction of it that goes to paying off principal versus interest changes through time and it just changes as dictated by this formula.

抵押贷款每月还款是固定的,但是其中本金和利息的比例关系,会随时间而改变,这是由这个公式所决定的

耶鲁公开课 - 金融市场课程节选