A / AN is an undefinite article.
We use the article A before singular, countable nouns starting with a consonant sound.
We use the article AN before singular, countable nouns starting with a vowel sound (a,e,i,o,u).
An before an h mute - an hour, an honour.
A before u and eu when they sound like "you": a European, a university, a unit.
If there is an adjective or an adverb-adjective combination before the noun, A /AN should agree with the first sound in the adjective or the adverb-adjective combination.
He was an excellent father.
A really beautiful car.
The indefinite article A / AN is used:
To refer to something for the first time:
A boy and a girl were waiting in the street.
Would you like a piece of cake?
I've finally got a new laptop.
To refer to a particular member of a group or class:
With names of jobs:
John is a teacher.
Mary is training to be an engineer.
He wants to be an artist.
With nationalities and religions:
John is an Englishman.
Kate is a Catholic.
With musical instruments:
Sherlock Holmes was playing a violin when the visitor arrived. (But we say "He plays the violin" to describe the activity. )
With names of days:
I was born on a Tuesday.
To refer to a kind of, or example of something:
This girl had a tiny nose.
The elephant had a long trunk.
It was a very old house.
With singular nouns, after the words "what" and "such":
What a shame!
She's such a beautiful girl.
Meaning 'one', referring to a single object or person:
I'd like an orange and two lemons please.
The thief stole a diamond necklace and a valuable painting.
In English, some nouns are considered uncountable such as: information, air, advice, salt and fun. We do not use A / AN with these uncountable nouns.
She always gives me
a good advice.
She always gives me good advice.
Notice also that we usually say a hundred, a thousand, a million.