How to use articles (3): a and an

A/an does not add much to the meaning of a noun. And we normally use a/an only with singular countable nouns.

1 one person or thing
   There is a car outside.

2 any one member of a class
   A teacher must like students.(=any doctor)

3 classifying and defining
   When we say what they are, what job they do, or what they are used for, we can use a/an to classify and define people and things.
    She's a teacher.
    A glider is a plane with no engine.

4 descriptions
   She's a nice girl.       That was a lovely evening.

5 when a/an cannot be left out
    We do not normally leave out a/an in negative expressions, after prepositions or after fractions.
      Lend me your knife. I haven't got a knife.(not I haven't got knife)
      You mustn't go without an umbrella.(not without umbrella)
      one third of a pound(not one third of pound)
    Also, do not leave out a/an when we say what jobs people have, or how things are used.
      He is a student. (not he is student)

6 the difference: a/an
   The choice between a and an depends on pronunciation, not spelling. An is used before a vowel sound, even if it is written as a consonant.
      an hour /En 'auEr/            an Mp /En em'Pi:/
      a university/E ju:nI'vE:sEti/    a one-pound coin / E 'wVn/
  Some people say an, not a, before words beginning with h if the first syllable is unstressd.
      an hotel (a hotel is more common)
      an historic occasion(a historic…is more common)