belt comes off

I'm guessing you mean the "serpentine" belt? Look for a missing plastic splash shield first. Next, check the spring-loaded tensioner pulley to be sure it's putting enough tension on the belt. Sight along all of the pulleys to see if the belt is peeking out on one of them. If it's not in perfect alignment with the parts that go around the other pulleys, one pulley is misaligned, usually due to a worn shaft or bearing. That happens most commonly to the tensioner pulley.