English Language & Usage Stack Exchange is a question and answer site for linguists, etymologists, and serious English language enthusiasts. It only takes a minute to sign up.
Sign up to join this community
For example:
Upon seeing a cup on my colleague's desk, I say, "you have a very big cup."
But the cup is already known to him (it's his and there is only one cup on his desk).
Regardless, the above sentence sounds more correct than "you have the very big cup."
Which is correct?
You use "a" when the object is a generic representative of a larger class, and the specifics of its identity are not important to you. "The" implies a specific individual object.
In this case you say "you have a cup on your desk," because there's nothing of particular distinction about that cup. If you said "you have the cup on your desk" it would imply that there was something we would both agree is distinctive and important about that cup.
On the other hand you would say "give me the cup on your desk" not "give me a cup on your desk", because in this case you want that specific cup, and there is only one that matches the description.
The way it works has nothing to do with what's "evident" to whom, nor according to whom.
It's linguistic, not philosophical.
An indefinite article is used the first time an object is referred to in a discourse.
After that, the definite article is used for subsequent references to the object in the discourse.
Referents become "definite", in other words, by being already loaded in working memory.
Until they're retrieved from storage, they're "indefinite".
This is the general rule only, understand; it is frequently superseded by literally dozens of other, specialized, individual, idiomatic rules for the use of articles.
If both of you know it, you could say "this cup" when referring to it.
