The indefinite article when we mention something for the first time

Question

I stumbled upon the following example sentence in a Russian article about English grammar (not articles):

There were a lot of people at a conference.

The use of the indefinite article does not seem correct to me: I suppose, we know what the conference it is since we know how much people were there.

In the comments, I found that it is not only me who noticed that. The authors give the following explanation: there is no enough context to claim that it is a specific conference. When we mention something the first time, we use the indefinite article.

The question is: is the use of the indefinite article justified in this case?

Answer 1

Yes it is justified if you're starting a story by introducing the characters and setting the scene. Here is one example of how this could go:

Diseases can spread quite suddenly and quickly. In 2015 at the New Orleans Convention Center, there were 500 people at a conference. The conference started at 8am. One of the attendees had the flu and by 3pm all the attendees were sick.

A lot of jokes start this way:

Two fishermen are drinking in a bar... A bunch of students were sitting in a classroom... Three nuns walk past a company picnic...

Answer 2

I don't disagree with CocoPop, but want to say to the OP that he's going about this subject backwards. The choice of article reveals how the speaker is thinking about the noun. Choice of article reflects the speaker's point-of-view. The facts of the situation are relevant only insofar as they are relevant to the speaker's point-of-view.

Consider:

Jane sees her boyfriend and her best friend, Ann, kissing. Later, she asks Ann incredulously

"Why would you ever kiss a boy I'm seeing?"

Jane knows which boy it is. But she's not talking about this boy, she's asking Ann about any boy she might be dating. She's asking Ann a general question, not a specific question. The specific question would be:

Why would you ever kiss the boy I'm seeing?

And even that specific question could be perceived in a general sense: "the boy I'm seeing" could be understood to mean "the boy I'm dating at the time", since the (reduced) restrictive clause "I'm seeing" doesn't necessarily mean "seeing now". The time reference could be "at any time".