2004 Hyundai Sonata battery problems

Hi there,

I have a battery problem with my Huyndai Sonata 2004. I bought this new car on Dec 04. Here are the problems I had with my battery.
1. On Dec 05, the car battery was changed.
2. On Nov 06, the battery terminals were cleaned up as there were corrison on them.
3. On Apr 08, alternator was replaced.
4. Sep. 2, 08, the battery terminal was cleaned up again as there were corrison on them again.
5. Nov. 21, 08, the car wouldn't start again. we dont know what is the problem.

It doesn't look like starter is a problem as the car starts when we jupmed it with another battery.

what should I check at this point?

A test result that is done by a service man today said ok. but I am not sure whether it is a battery problem. Should I change a battery?

I am wondering whether I need to ask a service center to find out whether something in my car is discharging the battery.

I am also wondering whether the alternator which was replaced could be a problem.

Thanks for your help.

Sue