When should I use the ideal gas law and not the combined gas law?
Ideal Gas Law: PV=nRT
Combined Gas Law:
The difference is the presence of "n" the number of moles of a gas, in the Ideal Gas Law. Both laws deal with pressure, volume, and temperature, but only the ideal Gas Law will allow you to make predictions when you vary the amount of gas.
So, if you are asked a question where gas is added or subtracted , it is time to get out the Ideal Gas Law . If the amount of gas remains constant and all you are doing is varying the pressure, temperature, or volume, then the Combined gas Law is your best option.