Notes On Indefinite Article - Usage - CBSE Class 9 English Grammar
Indefinite Article:
   “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.
An:
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

A:
 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

Summary

Indefinite Article:
   “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.
An:
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

A:
 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
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