Learn English – “At the table” or “at a table”

The definite article is not used in this expression.

When we assign an entity membership in a class we use the indefinite article, regardless of how ‘determinate’ the entity is, because it is not the only member of the class.

For instance, we ordinarily say “I own a ’57 Chevy”—that is, “The car I own belongs to the class ’57 Chevy”. You own only one car, it is entirely determinate, you introduce it into the conversation with the definite article; but it is just one of many ’57 Chevys. You would only say “I own the ’57 Chevy” if you and your hearers were looking at a group of cars and you were identifying your car as the only ’57 Chevy in the group.

Likewise, when you speak of a particular individual known to your hearer, you use a definite determiner in identifying or naming her—my wife—but the indefinite article in assigning her to a class: “My wife is a graduate student in mediaeval English lit.”

In the same way, when you say “What a car!”, you are not identifying the car but asserting its membership in the class of “cars to which attention should be drawn”.

We never say "What the X," because that utterance is not intended to identify which X you are talking about. It does not enjoin the hearer to "Look at that X", it observes that that X is an X of a certain class: it is an X which excites your admiration and deserves your hearer's attention.

As far as I know, the article the comes from the old English word for that (if something has a the in front of it, you can point at it). So, you can kind of mentally substitute the with that (or those for things that are plural) and if the whole thing still makes sense, then the usage is most likely correct. The indefinite article a/an, again, comes from the old English word for one. I usually think of it like this:

Volvo is a car. It is a car because it is just one of many possible cars that exist.

or

Volvo is a car. It is a car because it is one example of the nebulous idea of car.

Another example:

I'm gonna buy a car. I'm gonna buy just one out of many cars that exist.

With the, I think, we will typically be able to add some very specific information about the object we are talking about. For example:

I saw the man again. (The man that we met yesterday and his name was John.)

Where are the kids, honey? (The kids that are named Jimmy and Bobby because they are our sons.)

Compare that with examples using a:

I saw a man again. (maybe in my dream) (just a man, one of the class man)

What do kids like most of all, honey? (just kids in general, not our children or our future children)