IF function (?)

[an]

Hello!

For calculation of incomes, I have the formulaes:
(Thanks for Ron Rosenfeld)

A1 Initial value
B1 % annual interest
C1 Period (Months)
E1 =-$B$1/12*A1
F1 0

A2 A1+F1
E2 =IPMT($B$2/12;ROW()-2;$C$2-1;$A$2)
F2 =PPMT($B$2/12;ROW()-2;$C$2-1;$A$2)
G2 =E2+F2

and work fine.

Now, I would like to introduce a condition IF in
formulae / formulaes, when I make an amortization (Ex:
Colmumn "H") to reduce the value in debt (Column "A"), and
of course the respective monthly incomes but I'm with
difficulty.
Any idea well will be received.

Thanks in advance
an