SELECTIVIDAD -ERRORES EN LA REDACCIÓN ¿CUÁNDO USAR "A"," An" or "The"? DIFERENCIAS PRINCIPALES

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

Look at these examples:

the	a, an
The capital of France is Paris. I have found the book that I lost. Have you cleaned the car? There are six eggs in the fridge. Please switch off the TV when you finish.	I was born in a town. John had an omelette for lunch. James Bond ordered a drink. We want to buy an umbrella. Have you got a pen?

Of course, often we can use the or a/an for the same word. It depends on the situation, not the word. Look at these examples:

• We want to buy an umbrella. (Any umbrella, not a particular umbrella.)
• Where is the umbrella? (We already have an umbrella. We are looking for our umbrella, a particular umbrella.)

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.)

This little story should help you understand the difference between the and a, an:
A man and a woman were walking in Oxford Street. The woman saw a dress that she liked in a shop. She asked the man if he could buy the dress for her. He said: "Do you think the shop will accept a cheque? I don't have a credit card."