“A or An” is called the indefinite article, because it usually leaves indefinite person or thing spoken of.
For Example: A Doctor.
That is any “Doctor”.
The indefinite article is used before singular countable nouns, e.g.
A book, an orange, a girl.
Use of “A or An” or Indefinite Article
The choice between a and an is determined by sound.
Before a word beginning with a vowel sound “an “ is used; as,
An enemy, an inkpad, an orange, an umbrella, an hour, an honest man, an heir.
In above examples the words hour, honest, heir begin with a vowel sound, as the initial consonants h is not pronounced.
The article “an”is used by using this formula
Common noun + countable + singular + vowel sound
Before a word beginning with a consonants sound “ a “ is used; as,
A boy, a women, a yard, a horse, a hole,
Also a university, a union, a European, a ewe, a unicorn, a useful article.
At above the words university, union, unicorn, useful begin with a consonant sound, that of yu.
Similarly we say ,
A one-rupee note, such a one, a one-eyed man.
Because one begins with the consonant sound of w.
The article “a” is used by using this formula
Common noun + countable + singular
The Indefinite Article is used-
1. In its original numerical sense of one; as,
Twelve inches make a foot.
Not a word was said.
2. In the vague sense of a certain; as,
One evening a beggar came to my door.
3. In the sense of any, to single out an individual as the representative of a class; as,
A pupil should obey his teacher.
A cow is a useful animal