A/an
We use a/an when we talk about something and we don' t know which one, or it isn't important which one.
- I'd like a sandwich, please.
- What sort would you like? We've got cheese or chicken.
Have you got a laptop? (= any laptop. The speaker doesn't know which one.)
We use a/an when we talk about something for the first time.
- She's got a bicycle. The bicycle is blue.
We also use a/an when we are talking about someone's job and when we mean 'one ' .
- He's an actor.
- She's a teacher.
- a hundredpeople
- an hour
