Think of a chemical equation the same as you would a mathematical equation, both sides have to have the same number of each atom and the chemical symbols are the variables. First of all, you can NEVER change a subscript (small # written at the bottom). An example of a subscript is the 2 in H2O. It always referes to the # of atoms directly before it (left). There are 2 Hydrogen atoms in H2O and only 1 Oxygen, the 1 is "understood". Coefficients (large # in front of chem. formula) tell you how many complete "items" the are and is applied to everything written behind it (right). It's the same as distributing in math ex: 3H2O The coefficient 3 means there are 3 H2O molecules ie. "items" for a total number of H atoms of 6 (3 x 2) and a total number of O atom of 3 (3 x 1).
If a subscript appears after parentheses () such as Ca(OH)2 the 2 applies to the number of (OH). The 2 is distributed to ALL elements inside the (). If you put a coefficient of 3 in front ie 3Ca(OH)2 then the 3 means there are 3 Ca(OH)2 Total number of Ca=3, O=6 (2x1x3) and H=6 (2x1x3). In this example all of the "1's" are understood inside the () and for the Ca.
In short, all numbers get distributed either immediately before (left) as with subscripts or after (right) with coefficients. The ONLY time you add is if a '+' appears between 2 elements in the equation.
Practice, practice, practice!!!!