Read the following examples
I’m going to the post office to send these letters.
(we know which one, the one the speaker usually goes to)
Excuse me, is there a post office near here?
(any post office, not a particular one)
I don’t drink tea.
(any tea – I’m not talking about some specific situation and specific cup of tea)
Did you drink the tea that I made?
(we know which tea the speaker is talking about)
I didn’t like the film.
(we know which film – the one that the speaker has just seen)
Let’s go and see a film.
(the speaker doesn’t say which one)
Sam has two children, a boy and a girl. The boy is 5 and the girl 10. (mentioned for the second time – when we mention something for the first time, we are introducing it to the speaker, when we mention it for the second time, the speaker knows what / who we are talking about)
Who is the girl over there.
(we say which and where / not any girl, that girl over there)
I think that I left the keys in the kitchen.
(it’s obvious which kitchen and which keys – it’s clear from the situation)
People used to think the Earth was flat.
(there is only one)
I’m the tallest girl in my school.
(superlatives – because there is only one the tallest, best, fastest, etc)
Bill, I’d like you to meet Robert De Niro, the actor.