my battery gets a charge when it is jumped but it wont stay charged

WHEN U GET A JUMP-YOUR CAR GETS BUMPED TO START.WHEN U TURN IT OFF-IT GOES DEAD RIGHT??? YOU NEED A NEW ALT. DRIVE TO NEAREST AUTO PARTS STORE THAT DOES TESTING FOR FREE.THEY WILL ROLL A CART OUTSIDE RIGHT TO THE VEHICLE AND JUMPSTART IT AND CHECK EVERYTHING-BUT SOUNDS LIKE YOUR ALTY FER SURE ! GOOD LUCK !!!