We use a / an when the listener doesn't know which thing we mean
For examples:
- Jim sat down on a chair. (we
don't know which chair)
- Have you got a car? (not a
particular car)
The
We use the when it is clear in
the situation which thing or person we mean.
For examples:
- I took a taxi to the airport.
(= the airport of that town)
Where is the toilet, please? (=
the toilet in this house/building)
