Consider the standard joke form:

A man walks into a bar. The bartender says X. The man says Y.

Notice this has two indefinite articles and two definite articles. When do we use the indefinite article and when do we use the definite article?

In our sentence above, we’ve neither been introduced to the man nor the bar before. And so we use the indefinite article.

We can’t say “* The man walks into a bar” unless he’s been introduced before. Likewise we can’t say “* the bar” unless the bar’s been introduced before. For example,

Chris is one crazy guy! The man walks into a bar…

is fine if we take the man to be Chris. Similarly,

You know that bar on 52nd Street? A man walks into the bar…

works if the bar in the joke is the one on 52nd Street.

If we were telling a second joke, we could use the to indicate the man (or the bar) was the same but notice we’d have to use something like another and NOT a for introducing a second bar (or man):

Later, the man walks into another bar…

or

Later, another man walks into the bar…

Notice in our original joke, the third sentence starts “The man”. This makes sense because that man has already been introduced. We wouldn’t say “* The man walks into a bar. The bartender says X. A man says Y.” Even it were a different man, we’d probably use something like “Another man”.

But notice we did use the with the bartender even though he or she has NOT been introduced yet. The reason is our frame for a bar is that it has a bartender. The existence of the bartender has effectively been set up by us having a bar and that’s the bartender we want to reference so it’s not a completely new reference. Saying “* A man walks into a bar. A bartender says X” would be odd. Notice also that even if the bartender is a man, the following “The man says Y” is unambiguous.

Even if there were more than one bartender (certainly possible, although not prototypical for the frame) we’d have to say something like “One of the bartenders says X”.

This can be demonstrated with an example where we EXPECT multiple instances.

A man walks into a classroom. One of the students says X.

In this case, it would be odd to say “* A student says X” and even odder to say “* the student says X”. We want definiteness (because the classroom frame has already established the likelihood of a group of students and that’s the group we want to reference a member of) but because it’s a group, we need to say “one of” to call out an individual.

“One of the” calls out an indefinite member of a definite group.