Why my car does not start?

-I had to use the jumper cables, so when i did this the car started, but when I shut down the engine and by the time i want to start the car again it wont start, its only in a matter of seconds, but when i use the jumper cables with another car that supplies power to me then the car starts right away..is it the battery? starter? alternator?it could be any of those three things. my car had the same prob a yr back, i changed the alternator, then the battery and that didn't help and so i had to bring it in and it was the crankshaft sensor! If its anything sensor related, its really hard for you to diagnose that, you need a sensor tester....
you can look here, hope help.

http://www.samarins.com/diagnose/
restart your car-go to auto zone or similar auto parts store-they can test your battery-alternator and its free!!(always good) they also have code reader there and can read codes-all you are out to find out problem is your time-then comes the money thing
try a new alternator
your battery is done get a new battery