Some words start with a vowel letter but begin with a consonant sound, so we use
“a” before these words: a university /f ju:nivfsiti/, a usurer /f ju../
Silent letter “h” we use “an” before it: an hour, an honest child, an hour, an heir.

 The definite article:
Is pointing concrete aspect, or aspects from group of the same aspects: The boy
(certain boy), the boys. This aspect is often the representative of certain group of
aspects. The Definite article developed (like in German language der, die, das)
from the Demonstrative Pronoun “that”, by weakening of the pronunciation in the
stressed form. Its written form “the” has these pronunciations: /Tf/ before the word
beginning with consonant and /Ti:/ before word beginning with vowel and before
nouns that we want to point out.

1) A/an
 We use the indefinite article in two forms:
- “an” before a, e, i, o, u, silent h eg: an upcoming, an interview
- “a” before all consonants eg: a debut, a video
[Friendship 6/2000, page 14]
The
 We use the definite article in one form but in different pronunciation:
- the /Ti:/ before a, e, i, o, u, silent h eg: the elegance, the orders
- the /Tf/ before all consonants eg: the castle, the throne
[Friendship 9-10/2000, page 11]

2) A/an
 We use the indefinite article before nouns that we have not yet mentioned.
Eg: I received a letter from one of my… [Friendship 4-5/1999, page 25]
Virginia lawmakers passed a law. [Friendship 9-10/2000, page 6]
This is a wilderness park in the Yukon.[Friendship 6/2000, page 11]
The
 We use the definite article before nouns that we have already mentioned.
Eg: The next letter was from… [Friendship 4-5/1999, page 25]
The law was declaring slaves as property.[Friendship 9-10/2000, page 6]
The park itself has a Natural History Museum.[Friendship 6/2000, page 11]

3) A/an
 The indefinite article has its numerical meaning mostly before nouns, marking time, measure, weight or prize:
Eg: Rye`s maritime history goes back to the time over a thousand years ago. [Friendship 6/1999, page 17]
Winsdor is a small town just to the west of London.
[Friendship 6/2000, page 1]
A year in a day in Košice. [Friendship 6/2000, page 12]
They were running along about eighty miles an hour.