What happens if you just cannot or do not want to pay for a car any more that you have a loan on?

You wreck your credit. Even if you "give the car back", that is called "voluntary repossession" and it ruins your credit. Protect your credit rating. It's hard to establish, and even harder to re-establish... That's why insurance is so important. Wouldn't you just love to be making monthly notes on a totalled out car??