Tell me everything you already know about why we use indefinite articles.
I'll lay out the list of all conditions/contexts for “a/an” I can remember. But above all, there is a strong requirement: a noun must be countable and singular. Here are the contexts (I will use "thing" instead of "noun"):
1) A thing represents an instance of a class/set of objects. The speaker has a certain image of the object in his mind, but the listener doesn’t:
“I have a car (= a certain object belonging to the class of cars).”
a) The thing is mentioned for the first time = unknown and/or unidentifiable to the listener.
b) The thing is mentioned more the once, but its indefiniteness is maintained for the sake of logic and semantics:
“I have a car” - “What car?” - “A fast one (= a certain, known only to me example belonging to the set of fast cars).”
2) A thing represents an instance of a class/set of objects. The speaker doesn’t have a certain image of the object in his mind:
“I need a car (= any object belonging to the class of cars).”
a) The thing is mentioned for the first time = unknown and/or unidentifiable to the listener.
b) The thing is mentioned more the once, but its indefiniteness is maintained due to its genericness:
“I need a car” - “Why do you need a car?” - “I need a car because there is no public transport here.”
3) A thing represents a whole class:
“A lion is a dangerous animal.” It doesn't matter how many times it is mentioned in this sense. It’s indefiniteness is maintained due to its genericness.
4) We say
“there is a/an” when introduce new information or make a general statement.
5) General statements with
“have/has” require “a/an”:
“Every circle has a diameter.”
We say,
“This plane has a length of fifty meters,” but
“This plane has the length of a football field”, and
“The length of this plane is...”
6) We can say
“I’d like a beer/tea/coffee” instead of
“a mug of beer”, “a cup of tea/coffee”. These are examples of metonymy.
7) “A/an” with proper nouns
a) “a/an + name” = some unknown person/thing to the speaker:
"A John Smith (I’ve never heard of) called you yesterday."
b) “a/an + name” = an unusual or rare state of a person/thing:
"Yesterday I met an angry John."
c) “a/an + name” = the most characteristic feature of a person/thing (metonymy):
"He is an Einstein = He is a genius."
d) “a/an + name” = an artist’s/author's work (metonymy):
"The museum bought a (work by) Rembrandt."
8) I've remembered an exception to the first strong requirement. We can use "a/an" in the "adjective + numerical + plural noun" structure:
"We spent a good three years in Spain."
9) Neither the outer context nor the context within a sentence contains elements that make a thing definite: definite descriptions, previous mentioning, etc.
My point is that the sentence I’m asking about
does contain such an element, i.e. the definite description "
that was twice the dose typically prescribed."
Have you ever read, or considered reading, any literature on indefiniteness? There's quite a lot out there, and I think you're ready for it.
I've tried to read some academic papers, but it's a real challenge for me due to terminology. It would be difficult even in Russian, let alone English. But I keep on trying.
