We use them:

1.       With countable nouns in singular                  a dog    - a book    - a university  -an egg - an  orange 

2.      Before jobs                                                      Picasso was an artist.  -   Mum is a nurse.

3.       With quantifiers                                            There are a couple of films I want to watch.

                                                                             I have more than a hundred of photos in my mobile phone.

                                                                             Please, buy me a kilo of tomatoes.

                                                                             A thousand of people came to see the show.

4.      With expressions which talk about speed or frequency. 

                                                                            Jim drove 100 kilometers an hour.

                                                                            We study art three times a week. 

    (!) We don’t use a/an with uncountable nouns. Instead we use some.

               some sugar / some water / some furniture / some information / some advice / some luggag 

We use it:

1.       To talk about something specific that has already been mentioned. 

                                                    We saw a film last night. The film was funny.

2.      With an adjective to talk about a group of people.  

                                                The young like to have fun. 

                                                Did the Chinese invent ice-cream?

3.       With the names of rivers, seas, oceans, lakes and mountains(οροσειρές).

                                               The Nile is the longest river.

                                               The Mediterranean Sea is in Europe.

                                               The Atlantic Ocean – The Loch ness lake- 

4.      With the words      the sun     the moon  the sky   the earth   the world   the space

5.      With countable nouns in singular to talk about a kind of animal / instrument / machine

                             The lion is the king of the jungle. 

                             The dolphin is a very intelligent animal.

                              Janet plays the guitar.

6.     With names of people or places in plural                     The United States 

                                                                                                      The Himalayans

    NO ARTICLES before activities, countries or languages

                     I like tennis.                    Malaysia is in Asia.                          Greek is more difficult than English.