a/an the no article

to express general idea: no specific

Jack has a computer. (no brand) / He has an apple.

To express one (quantity)

I want a pencil = I want one pencil.

Se usa para referirnos a una cosa por primera vez. La próxima vez que nos refiramos a esta cosa, utilizaremos el artículo determinado.

I live in an apartment. The apartment is big.

Se usa para hablar del oficio de alguien o de afiliación política.

Maria is a doctor. Juan is an architect. I am a democrat.

Used when the reader knows which noun is being referred to because the noun has been mentioned before.

Some schools have a career counsellor. The counsellor has an important role in preparing students for work.

Used with unique phenomena which are easily identified.

The sun, the moon, the Vatican, the prime minister.

We don't use an article:

When we refer to all members of a group. For example:

Tigers are endangered.

With uncountable nouns when they refer to general concepts.

Water is essential for life.

Names of people, books, movies, plays (unless it is part of the title). Proper Nouns

Avatar is a science-fiction movie.

Many geographical features: towns, cities, states, countries, lakes, single islands, continents, mountains.

Melbourne is in Victoria.

Sports, games and meals.

He plays football. I like chess. They’re serving fried chicken tonight.