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Explanation
An article describes the Noun. It is used before the noun.
There are two types of articles in the English language. 
The is the definite article in English, which indicates that the noun is specific.
The speaker talks about a particular thing. 
The cat sat on the couch.
A and An are the indefinite articles in English, which indicate that something is not clear, obvious or exact. 
The cat sat on a couch.
A is used before a word beginning with a consonant sound (a boy, a cat, a dog, etc.).
An is used before a word beginnig with a vowel sound (an apple, an elephant, an umbrella, etc.)
We pronounce the article theas /ði/ in front of words that start with a vowel sound:
the apple, the evening, the eye, the example, the oak, the ears, the arm, the idea, the egg.
We pronounce the article the as /ðə/ in front of words that start with a consonant sound: the car, the cat, the sword, the man, the girl, the table, the kitchen, the book, the door.
Remember:
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We use a and an only before a singular noun. We can't use a and an before a plural noun.
A and An = one (an apple = one apple)
Ø Do not use articles when you talk about things in general.
    I like birds. I love music.
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Watch out for the articles in the collocations below:
a lot of, a great many, have a good time,
go for a walk, go to bed, go to school, go to church, go to the theatre,
in the morning, in the afternoon, in the evening, at night.