When mortgage payments are made in what way does the interest portion change each month and why?

Each month, the interest portion of the payment decreases and the principal portion of the payment increases. The interest decreases because the outstanding principal balance decreases each month as payments arev made. At the beginning of a loan, the interest portion of a payment is large and the principal is small. Towards the end of the loan, the interest portion is small and the principal portion is larger.