Difference between a/an and the

Very simply:

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

Compare:

• 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.)