The words "a" and "the" mean two very different things when referring to objects.
"The car drove silently down the road because it is electric."
In this sentence "the car" refers to a specific car. There is once specific car that is electric and that drove silently down the road.
A car drove loudly down the road because it had a large gasoline engine."
In this sentence "a car" means any car. A non-specific car drove loudly down the road.
Imagine you are sitting on a park bench with a friend. You see a car and want to comment on it. You would say "the car" or "that car" because you are referring to a specific car. Now let’s say you want to make a general comment about cars. You would say "a car" or "cars" because you are speaking generally, not specifically.
Look! The car that just drove past is super cool!
A car has four wheels and is usually powered by an internal combustion engine that runs on gasoline.
When your teacher (unclearly) corrects you by saying "one is here and one isn’t," she means that when you say "the car" it means you are talking about a specific car that exists as opposed to the general "a car" which refers to all cars or the idea of a car.