A word which pinpoints another word in the form of Noun, in a definite or indefinite form is called an Article. It usually comes before a Noun. Example, A chair, THE magazine, AN aeroplane. These three a, an, the represent the parts of speech, Article.
AN is used before Nouns which begins with vowel sound I.e, A,E,I,O,U.
THE is used when we are being specific about a noun. For example, Roopa said, “ Let’s go back to our room and find THE key.” Here, Roopa was specific about the key that she had to find hence, we used THE before key.
Whenever we are referring to a single Noun the Article A is used.