What are the differences between a, an and the?
The is used for the one thing that we know, and a and an are used for one thing among many.
And now for an illegal example
I want to steal a car. I don’t know yet which car, but I want to steal one car among many. Now I choose a car, so I decide to steal it. I break into the car, and we say the car because we know which car. Then I start the engine (one engine, and we know which engine). Now I steal the car.