articles: the difference between a/an and the

articles: the difference between a/an and the

a/an just means 'one of a class'
the means 'you know exactly which one'.

A doctor must like people. ( = any doctor, any one of that profession) My brother's a doctor. ( = one of that profession)
I'm going to see the doctor. ( = you know which one: my doctor)
I live in a small flat at the top of an old house near the town hall.
(a small flat: there might be two or three at the top of the house — it could be any one of these.
an old house: there are lots near the town hall — it could be any one. the top: we know which top: it's the top of the house where the person lives — a house only has one top.
the town hall: we know exactly which town hall is meant: there's only one in the town.)