The is used before countable nouns to talk about something the listener or reader knows about.
- I saw the moon last night.
(There is only one moon, so the reader knows which moon.)
- The capital of France is Paris.
(France has only one capital, so the reader knows which capital.)
- There is some milk in the fridge.
(I only have one fridge, so the reader knows which fridge.)
The first time you talk about something use a or an. The next time you talk about it use the.
- I want to buy an umbrella.
(There are lots of umbrellas to buy, so we don’t know which one.)
- I want to buy the blue umbrella we saw yesterday.
(We know which umbrella – the blue one – so the.)
- There is a woman in reception.
(We don’t know which woman it is because we have never seen her before.)
- The woman from the bank in reception.
(We know which woman it is. The woman from the bank that we have seen before.)
- A man in a white t-shirt is riding an orange bike. The man is looking for a place to park the bike.
(A man, a white t-shirt, an orange bike – first time we talk about these things.)
(The man, the bike – we know about these things. A place – first time we talk about this thing.)
- I was on holiday last week. The hotel was very good.
(We say The hotel because the reader knows, or can understand, it is the hotel I stayed in on my holiday.)