Indefinite Article has two forms – ‘A’ and ‘An’. Which of these two forms will be used before a certain Noun depends upon the spellings, sounds or pronunciation of the Noun under reference.
Indefinite Article Forms

The following are its rules –

  1. Article ‘A’ is used before Nouns of which the spellings begin with a consonant. As – a boy, a table, a house, a book.

  2. Article ‘A’ is also used before words which begin with a vowel but are pronounced as with a consonant. As – a European, a unique chance, a one-rupee note, a usual sight, a universal truth.

    – Such other words are – University, utensil, uniform, union, unity, unit, united, unitarian, useful, one.

    Note :-
    You will see that all these words begin with the sound of Y and W (which are consonants) and not with the sound of any vowel.

  3. Article ‘An’ is used before those words which begin with a vowel (a, e, i, o, u). As – an enemy, an ox, an inkpot, an ass.

  4. Article ‘An’ is also used before those words which begin with silent/mute ‘h’. As – an hour, an honourable man, an honest person.

  5. – Such other words are – heir, heiress, heirloom, honest, honesty, honorarium, honorary, honour, honourable, hour, hourly.

    Note:–
    Earlier humble, humility, hotel, hostel, hospital were also supposed to begin with silent ‘h’ and article ‘an’ was used before them. But now, these words are pronounced with the consonant sound of ‘h’, and therefore article ‘a’ is used before them.

  6. Some Abbreviations are there which are pronounced with a Vowel sound and therefore article ‘An’ is used before them. As – an M. A.; an M. P.; an M. L. A.; an F. I. R.; an S. D. O.; an H. M. T.; an R. T. O. etc.

  7. Note:–
    If the above noted Abbreviations are used in their full form, they take article ‘A’. As –
    • A Member of Parliament (an M. P.)
    • A Master of Arts (an M. A.)