Finding difficult to calculate answer after taking mod?

I have been practicing taking modulo for answers but not understanding when to add mod and when to substract it from the answer.
for e.g. ans=(ans-1+mod)%mod;
why it is so?

Remember that the answer is expected to be from 0 to mod-1 (inclusive) but the modulus operator ("%") doesn’t guarantee this.

For example, (-12 % 10) == -2, which is not what we want.
We would have to add twice the modulus to get our answer in the correct range:
(-12 + 2*10) % 10 == 8 (as expected)