Apply the combined gas law when pressure, volume, and temperature all change at once, and apply the ideal gas law (PV = nRT) to solve for pressure, volume, moles, or temperature.
The combined gas law merges Boyle's, Charles's, and Gay-Lussac's laws into one relationship for situations where pressure, volume, AND temperature all change at the same time (only the amount of gas stays fixed): P1V1/T1 = P2V2/T2. Each of the three simpler laws from last week is really just this same equation with one property held constant and canceled out.
The ideal gas law goes a step further, relating all four properties of a gas -- pressure, volume, amount (in moles), and temperature -- in a single equation: PV = nRT, where R is the ideal gas constant, 0.0821 L*atm/(mol*K) (matching units of atm for pressure and L for volume). Unlike the other gas laws, the ideal gas law describes a gas at one single moment, rather than comparing a "before" and "after" -- which makes it the right tool whenever a problem gives three of the four properties and asks for the fourth.
A useful special case of the ideal gas law is STP (standard temperature and pressure: 0 degrees C, which is 273 K, and 1 atm). At STP, exactly one mole of ANY ideal gas occupies 22.4 L -- this molar volume is worth memorizing, since it turns mole-to-volume gas conversions at STP into a single multiplication or division, without needing the full ideal gas law equation at all.
Mastery looks like: They correctly apply the combined gas law when multiple properties change at once, and correctly apply the ideal gas law to solve for any one of pressure, volume, moles, or temperature.
Common stumbles: Using R with mismatched units (for example, pressure in mmHg without converting to atm first); confusing STP's molar volume (22.4 L/mol) with Avogadro's number, which are two entirely different constants.