Indefinite Article (A/An) shows indefiniteness and Definite Article (The) shows definiteness.
Articles (A, An, The) are used before Nouns under some rules. When a Singular Countable Noun is used for the first time, Article A or An is used before it. But when a Countable Noun, whether Singular or Plural, is used for a definite thing, or refers to a thing or person that has already been referred to earlier, Article The is used before it. Thus when a Singular Countable Noun is used for the first time, Article a or an is used before it, but when the same noun is repeated in the same sentence or in the subsequent sentences, Article The is used before it because then it becomes a definite thing already referred to earlier. As—
1. I have a book. The book is very informative.
2. I have a book, though the book is not very informative.
3. There was a house. The house was very large.
4. There was a house and the house was very large.
Singular Indefinite Noun A or An
Singular/Plural Definite Noun The
Forms of Indefinite Article
Indefinite Article has two forms—A and An. Which one of these two forms will be used before a certain Noun depends upon the spellings, sound or pronunciation of the Noun under reference. The following are its rules—
(i) Article A is used before Nouns of which the spellings begin with a consonant. As—
A boy, a table, a house, a book.
(ii) 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 or w (which are consonants) and not with the sound of any vowel.
(iii) 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.
(iv) Article An is also used before those words which begin with silent / mute h. As—
an hour, an honourable man, an honest person.
Such other words are—heir, heiress, heir-loom, 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.
(v) 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.
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.)