THE ARTICLES

A, the indefinite article comes before words beginning with a consonant or a consonant sound. A becomes an, when it is followed by words beginning with a vowel or a vowel sound.



a mouse	a new school	a university
an elephant	an old school	an umbrella
an hour*	an honest*man



There is no plural form of the indefinite article. The plural of a dog is dogs, without an article.


The, the definite article comes both before words with a consonant or a consonant sound and a vowel or a vowel sound.


It is used with both singular and plural nouns.



the town	the towns	the new school	the union	the house
the eye	the eyes	the old school	the Underground	the hour*



*In a few words, the initial 'h' is not pronounced: hour, honest, honour, heir, and their derivatives (hourly, honesty, etc.)


Note
a or one? Think of one as a number, in contrast with two, three, and so on. One also contrasts with another and the other(s).
e.g.
There is more than  one way to live in harmony.
There are three telephones in the office. I've got one, and the boss has got the other two.


The uses of the articles: the or a(n)?


We use a(n) when the object is not specified; we use the when the object is specified. (To specify is to say which particular one you mean out of a group of objects.)



A Not known and not specified a(n)

B Known but not specified a(n)

C Known and specified the



A
I always like to have a book by my bedside.
--> any, we don't know which one


B
I received a book about gardening for my birthday.
--> a particular one, but not (yet) specified


C
"I have a book about carpentry and a book about gardening. Which one do you want to see?"
"Could I see the book about gardening, please?"
--> a particular one, and specified


Source: Allsop, Jake. 1990. Cassell's Students' English Grammar. Indonesia: Binarupa Aksara.