What is the difference between a coefﬁcient and a subscript?
The subscript is the amount of atoms of the element in that molecule. The coefficient tells you how many of that molecule you have.
The coefficient tells you how many molecules of that substance there is.
The subscript tells you what the substance it. It tells you the the amount of each element there are in the molecule. Changing it would change the substance itself.
Whereas simply changing the coefficient means you're changing the amount of molecules.