Dryer makes loud humming noise but drum doesn't turn when I push start.

Check to see if the drum will turn by hand. If so, leave the door open but push the light switch button down and start the unit. Once you hear the humming sound, manually turn the drum with your hand to start. If the drum turns normally, you have a bad motor and needs to be replaced. If the drum does not turn, something is jamming the motor and needs to be disassembled and inspected.
Good luck