the definite article (when we are talking about something or someone specific).

Rule 1:
We use 'the' when we are talking about something specific.

for example:
Tim sat on a chair. (perhaps one of many chairs in the room)
Tim sat on the chair nearest the door. (a specific chair)

Paula is looking for a job. (not a specific job)
Paula got the job she applied for. (a specific job)

Rule 2:
When it clear which which thing or person we are talking about.

for example

Can you turn the light on please? (the light in this room)
I took a taxi to the station. (the station in that town)
I'd like to speak t the manager please. (the manager of that shop)