My question is about expressions of the form "A of B" when uniqueness of A is guaranteed. Are the following four sentences all okay?
"Is ABC a capital of a country in South
America?""Is ABC a capital of some country in
South America?""Is ABC the capital of a country in South
America?""Is ABC the capital of some country in
South America?"
I suspect 3 and 4 are okay because there can be only one capital for each country. On the other hand, I suspect 1 and 2 are okay because there are many cities which can be said to be a capital of a South American country.
Best Answer
All four sentences are grammatical.
Answer: Yes, as a matter of fact, it is; it's the capital of Peru.
Answer: No. It's a city in Brazil, but it's not the capital.
Answer: Yes, Quito is the capital of Ecuador.
Answer: Yes, it's the capital of Venezuela.
Why does the definite article work as the first article? Because (ignoring the Bolivia's geopolitical peculiarity for a moment) each country has one capital city – the capital of the nation. Therefore, we can ask, "Is Los Angeles the capital of a country in South America?"
Why does the indefinite article work as the first article? Because you are asking about several countries at once – all the countries in South America. Therefore, we can ask, "Is Los Angeles a capital of a country in South America?"
If you narrow it down to one country, then you should use the definite article (the):
Answer: No, Santiago is the capital of Chile.
unless you were asking about the unusual case of Bolivia; in that case, the indefinite article could be considered acceptable:
Answer: Yes, it's one of the capitals; Sucre is the other.