Indefinite Article
 
a / an
 
 

Beginner
CEFR levels A1 (Breakthrough) and A2 (Waystage)

 a  is used when the pronunciation of the next word starts with a consonant (b, c, d, f, g, h, j, k, l, m, n , p, q, r, s, t, v, w)

           a book                        
           a car                                       
           a house                                 
           a computer                        
​           a hobby 
                 
           a uniform                               
           a university                           
           a one-man show                  
           a one-armed bandit             

 an  is used when the pronunciation of the next word starts with a vowel (a, e, i, o, u)

            an egg                                     
            an umbrella                          
            an oven                                  
            an apple                                 
            an inch                
           
            an hour                                  
            an honest girl                      
            an MP                                    

The indefinite article is used:

• before singular countable nouns, when it is unclear, or unimportant, which one we are talking about.

             I have got a book.
              This is a very good story. 
              She is a very difficult child.

• when referring to something in general, but not a specific case.

              When a man is tired of London, he is tired of life.
              A horse is an animal one should be careful with.

• when referring to someone's profession, belief, title or nationality.

              He is  carpenter.
              She is a Catholic.
              She is a duchess.
              He is an Irishman.

Intermediate
CEFR levels B1 (Threshold) and B2 (Vantage)

The indefinite article is also used when referring to frequency:
            twice a day                                                         
             four times a month                                           
 

 The indefinite article is not used when only one person at a time can fulfill a particular position.
                She is Duchess of York.                        
            He is President of the U.S.A.