how do you know what/which roman numeral to place between the cation and the anion of metal-nonmetal substances?

Answer 1

if the sountance is hard that is roman numeral.

Answer 2

Tammy is incorrect with that answer. The correct answer is that the Roman numeral is the oxidation state of the cation. For example, in CuSO4 it is copper(II) sulfate. The old name was cupric sulfate.
Another example. Cu2O is copper(I) oxide. CuO is copper(II) oxide.
FeSO4 is iron(II) sulfate.
Fe2(SO4)3 is iron(III) sulfate.