I think the BBC World Service article on 'it' and 'there' may help explain it.
The way I understand the situation, you were talking about a homework assignment (which you already had the questions for) and someone said
There are the answers on the website.
and we know that is wrong, but it's not really clear why it's wrong.
When we use there + be + noun phrase, we are saying something exists (or doesn't exist when we use the negative). We use indefinite articles, pronouns, or determiners with this form. For example,
There is some milk in the fridge.
There is a clue in the library.
There are bats in the belfry! (we don't know how many, but it seems like a lot!)
There is something coming and I think it wants to hurt us! (to paraphrase a character in a video game).
When we know that the subject exists because we've already identified it, like "the answers to the homework assignment", we don't use this form. I don't have to tell you the answers exist, because you already know they do if we're using the definite article. So, we would say:
The answers are on the website.
We could also say
There are some answers on the website.
Are there any answers on the website?
*Are there the answers on the website?