In a chemical equation, changing the subscript in a chemical formula for a substance changes what aspect?

You change the identity of the substance!

The subscript in a chemical formula is the number written next to the element at the bottom part. For example, the chemical formula of water is H₂O. The subscript of H is 2, while the subscript of O is 1. The subscript represents the number of a certain element in one particle of the compound. So, if you change the subscript, you also change the number of a certain element per compound. In other words, you change the ratio.