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.
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)