The article

2. Article Usage in English


2.1 The Indefinite Article a, an

For better or for worse, English is blessed with articles. This causes a considerable amount of confusion for speakers of most of the world's other languages, who seem to get on rather well without them. The good news is that English began dropping the complex case systems and grammatical genders still prevalent in other European languages a very long time ago. Now we are left with just two forms of the indefinite article (a & an) and one form of the definite article (the). Perhaps more than anything it is the transition from being a language with synthetic structure to one which is more analytic that has helped gain English the kind of unrivalled worldwide acceptance it enjoys today.

Even between British and American usage one finds subtle differences in nuance or emphasis. For example, Americans usually say someone is in the hospital, much as they could be at the bank or in the park. To the British this sounds like there is only one hospital in town or that the American is thinking of one hospital in particular that he or she patronizes. The Brits say an ailing person is in hospital, just as they would say a child is at school or a criminal is in prison. This is because they are thinking more of the primary activities that take place within those institutions rather than the buildings in which they are housed. If, however, you are merely visiting one of these places, you are at the hospital, at the school or at the prison – both British and Americans agree here that what we have in mind is the building itself.
We use a/an in such cases:

• 
  To facilitate pronunciation, a is used in front of any word that begins with a consonant or consonant-like vowel sound.
  

Conversely, an is put in front of any word that begins with a pure vowel sound or a mute 'h'.
Our town has a theatre, a university, a large park and a conference hall.
Many Chinese still believe an Englishman always carries an umbrella.
It's an old custom.
It's a strange old custom.

• 
  Note that spelling is not a reliable indicator of when to use a or an!
  

The coastguard received an SOS.
He spent an hour standing in line.

• 
  The indefinite article a/an is placed in front of a countable noun that is being mentioned for the very first time. Once introduced, all further references to it can be preceded by the definite article the.
  

I have two cars: a Ford and an Audi.