The two articles a and an are called indefinite articles because they do not refer to any particular person , animal ,thing,or place they refer to things in a general manner  .  
     the indefinite articles a and an are used for referring to people or things which we are mentioning for the first time. after that, we have to use the definite article the to refer to them . 
   the definite article the is used for people , animals,things,and places we already know about.