The form  «a»  is used before a word beginning with a consonant, or a vowel sounded like a consonant, i.e. the sound of [yü]:  

EX:  a movie  /   a costume  /   a united team   /  a useful script

The form  «an»  is used before words beginning with a vowel (a, e, i, o, u) or words beginning with a mute h:  

EX:  An unknown actress    /   an actor  an horror movie an action movie

The indefinite article  «a»  or  «an»  is used: 

-    before a singular noun that represents no particular person or thing,  

EX:     The film maker needs a holiday.

          There is a line up at the theatre entrance.

-     before a noun representing time, distance, or numerical  expressions:

          (a couple, a dozen, half a dozen, an eighth, a quarter, a hundred,  

          a thousand, a million, a lot of, a great many, a great deal of).  

EX:     It took more than a month to make the Titanic movie.

         The Titanic movie lasts at least a hundred minutes.

          I have a dozen cassettes.

-     with a noun complement, including names of professions,  

EX:     He became an actor.

-     in expressions of price, speed, ratio, and before such, what, quite, as,…  

EX:       Demi Moore might exercise three times a week.

Matt Leblanc is such a popular comedian.

What a great movie!

Marilyn Monroe is quite a model as a good actress.

The indefinite article is not used:

-     before plural nouns,  

EX:       a scene = scenes (plurial form)

-     before uncountable nouns (advice, information, news, baggage, luggage,

       furniture, knowledge).  They are often preceded by some, any, a little,

       a lot of, a piece of, etc.

Ex:      She has a good knowledge of drama.

           My teammate will give me a piece of advice.

-     before abstract nouns (beauty, happiness, fear, hope, death, etc),

Ex:      Money does not bring happiness.

           Ridley Scott does not suffer from a fear of the danger in Gladiator movie.

-     before names of meals, except when preceded by an adjective.

Ex:    “Brad Pitt had a good lunch with ten friends at Ivy at the Shore on Ocean Ave.

          in Santa Monica.” *

The definite article  THE

It is the same for singular and plural and for all genders:

Ex: The curtain, the curtains  /  the actor, the actors   /    the actress, the actresses

The definite article  «the»   is used:

-      before nouns of which there is only one,  

Ex:   the Canada     /    the Titanic    /     the moon    /    the weather

-      before a precise person or thing,

Ex:   Every month I buy the Movieline Magazine.

        The girl I saw in this movie is pretty.

-     before a  noun which by reason of locality can represent only one particular thing,

Ex:  Please pass me the bread (the bread is on the table).

-    before superlatives and  ordinal numbers,

Ex:  The best site about movie scoop is movieline.com.

       It is not the first time I watch Air Force I.

-     before singular animal or thing,

Ex:  The kangaroo Skippy was very friendly.

-      before a category of persons,

Ex: The poor do not subscribe to the Indigo channel.

-      before certain proper names of oceans, seas, rivers, group of islands,  

       chain of mountains, nations, plural names of countries, deserts,

Ex:   the Atlantic   /   the Alps    /    the Sahara   /    the U.S.A.

-      before a musical instrument,

Ex:  Lucy Liu plays the accordion.

-      before cardinal points if followed by of,

Ex:  What actor is living the west of Spain?

-      before a body part,

Ex:  The foot of Antonio Banderas was broken at age 14.

Omission of THE

-      before home and before a visited place or used for a primary purposes.

Ex.  (market, college, school, hospital, court, prison, work, bed, sea…)

-      before a game, a sport, an illness, a season, a language, material in general, days 

        of  the week, a school subject, abstract noun.