Updated: Sep 10
It all depends on your point of view.
Consider these sentences:
"I was waiting at the bus stop and a bus drove past."
This bus could have been going to any destination and was not the bus that the speaker was waiting for.
The statement uses the indefinite article a which is non-specific.
"I was waiting at the bus stop and the bus drove past"
By using the word the in this sentence we know it was the bus the speaker was waiting for. The statement uses the definite article the which is specific.
Looking at the question in the picture: Can you see a car, or the car?
It all depends on the situation.
Imagine you are trying to cross the road. You would be looking to see if you could see a car. It wouldn't matter what car it was as long as it was safe to cross the road.
Now imagine you have been shopping and cannot remember where you have parked your car (this has happened!). You would be looking around to find the car. In this case it is a specific car - your car!
Use the indefinite article a (or an) when you do not need to be specific (or clear)
Use the definite article the when you need to be specific (or clear) about the object.