How do subscripts represent the charge balance of ions

You use superscripts (not subscripts) to represent charge on an ion. Unfortunately, that's difficult to do on this page without special formatting (and I'm too lazy to do that). So, I write Cl- or Cl(-). You just have to remember that charges (+) or (-) are superscripts. Subscripts represent numbers of atoms. So, in sulfuric acid, H2SO4, there are two hydrogen atoms and 4 oxygen atoms. These would be written as subscripts if I weren't so lazy.

Some ions are doubly or triply charged: Sulfate: SO4(-2). Carbonate: CO3(-2). Magnesium: Mg(+2).

When compounds are made from ions, the charges must balance to create a neutral molecule. For example, Mg(+2) combines with two Cl(-) to form MgCl2 (the 2 in this case is a subscript). The subscript indicates that there are two chloride, Cl(-), ions contributing to the compound.