What would cause an engine to stall?

Engines need 3 elements to run. Air, fuel, and ignition. Confirm all the components in the fuel delivery system are working correctly: Pump, filter, injectors. Then confirm you are getting spark to your spark plugs. If no spark, check the coil, Ignition control module, pick-up coil (if applicable), and distributor cap and rotor. If you do have spark, make sure the spark plugs are not fouled. Engines almost always have enough air to run unless the air filter gets extremely plugged. If you have all 3 elements at the times it stalls, the air-fuel mixture could be getting affected by something like a vacuum leak, bad sensor(s) (O2, MAP, MAF, TPS and possibly others depending on the car) or physical buildup in the air delivery components. Your question is extremely general. Turning the key off will also cause an engine to stall.