calculating payments in loan calculator

I am trying to understand the formula:
=if(D16="";"";ROUND(PMT(D9/PAYMENTS_PER_YEAR;D16;-D8;2))
this is related to HOW TO CALCULATE PRINCIPLE AMOUNT OF A LOAN INSTALLMENT.
in loan calculators, they ask us to put:
loan amount, interest rate, number of payments
then, they calculate the interest (I know how.
and then they calculate the principle payment (I need to know)
then they add both together (interest + principle) to get to the monthly payment.
please assist me with thank
adel

Use =PV() function