Question

Why can't you change the subscripts in a formula in order to balance a chemical equation?

Answer 1

The subscripts determine what the chemicals are, you have to change the amount of the reactants/products rather than change what the reactants/products actually are
eg O2 is oxygen gas, if you wanted 4 of them to balance it it would have to be 2 O2 otherwise it wouldnt be oxygen gas any more :)

Answer 2

Answer: you cannot change the subscripts in a formula in order to balance a chemical equation because the subscripts indicate the number of each kind of atom in the formula,

Each compound has an invaraible chemical formula which tells how many atoms of every kind constitute the compound.

Varyng the subscripts is a modification of the chemical formula, meaning that you end up with a different compound.

On the other hand, by varying the coefficients, you only modifiy the number of unit formulas (number of molecules of the compound) that intervene in the reaction, which is what you want to indicate by balanceing reactants and products.