This is how most people would explain it:
I want to buy a new car. You want to buy some car. You don't know which one exactly, Toyota or BMW, which color, and so on.
The car parked near the baker's shop is red. Specific car. That one car.
Hey, Joe. You know, I bought a new car yesterday. I bought some car.
As I know, the third example is correct (as well as the 1st and 2nd ones), but it confuses me. When I buy a car, I buy the specific one. For example, it could have a small scratch on its door.
Why we use the indefinite article there?