A - AN - THE

A - AN :

A and an are indefinite articles used to refer to a singular countable noun.

An indefinite article means that we do not know which one, or it is not important 

to know it. 

Which one to use:  'a' or 'an'?

The rule states that “a” should be used before words that begin with consonants

(b, c ,d etc.) while “an” should be used before words that begin with vowels (a,e,i, etc.).

It should be noted, however, that the usage is determined by the pronunciation and

not by the spelling, and this includes abbreviations and acronyms.

To simplify, one uses 'a' before a word that begins with a consonant SOUND,

and 'an' before a word that begins with a vowel SOUND. 

A is used before :

• a consonant (b, c, d, f, g, etc.)         a car/a hotel

• a vowel that is pronounced like "yu"            a European/a university

• the vowel 'o' when it has a "w" sounda one-way street

An is used before :

• a vowel  (a, e, i, etc.) :         an animal/an elevator 

• an unaspirated 'h':   an hour/an honest man

• abbreviations starting with a vowel sound:an MBA ('em' sound)

THE :

The is a definite article used to talk about something specific. 

• The town where Julie lives is very big.
• What book is Julie reading? She's reading the book Tom gave her.

The is also used to refer to:

• Rivers, seas, oceans :
  • the Mississippi river, the Mediterranean sea, the Atlantic ocean
• Nationalities :
  • the British, the Americans, the Japanese, the Chinese, etc.

NO ARTICLE :

No article is used in generalisations:

• I like music
• Caviar is expensive.
Exceptions :
• I watch television but I listen to the radio and I go to the cinema.
• I don't play tennis but I play the piano, the guitar, etc.

No article is used for place names (towns, countries, mountains) :

• London, Spain, Mount Everest
Except if plural :
• The Greek islands, The United States, The Alps, The Himalayas