Why would a car not start with your key but start easily with a jump start?

Your car could have a dead battery. Your key opens your electical system only while a jump allows you use the other car's electical system (good battery and all). It could also be a bad starter. Autozone can check your starter, battery, and alternator for free. that happend on our 94 ford explorer and it turned out to be a bad wire connecting to the starter!