1. to refer to something which is known. the list, Dad’s in the kitchen
2. to talk about something which has been mentioned before. e.g. I’m going to see the film you told me about last week.
3. when there is only one of something. e.g. The president said that the country would never forget what they had seen.
4. in a superlative expression (and before only… and the first, etc.) e.g. Tom’s the only person I like at work.
5. with names of buildings, plural names (eg. countries, rivers, oceans, etc.). e.g. I went up the Empire State Building when I was in New York.
6. with countries whose names include a noun, e.g. Have you been to the Czech Republic?
7. to refer to different nationalities. e.g. People think the British are very reserved
8. with an adjective when we want to refer to a group of people (we don’t need to include people after the adjective). e.g. The government should do more to help the unemployed/the old/the young, etc.
9. to refer to something which is defined after (eg. with relative clauses). e.g. We went to the house where Shakespeare was born.

We use the indefinite article (a, an)…

1. to refer to something for the first time. a nice comment e.g. Have you heard that Catherine’s bought a cat?
2. to refer to something unspecific. e.g. How often do you use a computer?
3. to talk about what something is or what job somebody does (to classify). Ruth is a biologist. Pepper is a spice.

We use the zero article (none/niguno)…

1. with plural countable nouns (when we are speaking generally) good things I don’t like dogs.
2. with uncountable nouns (when speaking generally). e.g. I love red wine.
3. with languages and most place names. e.g. French is spoken in some cities in Canada.
4. with certain expressions: meals, transport, places (eg. school) e.g. He went to bed after he had dinner.
5. with abstract nouns, e.g. Love can be very difficult. Unemployment and crime are the two biggest problems the government is facing.

Source: “Framework: Student Book Level 3″ Richmond Publishing; “Framework Reference Guide Level 3″ Richmond Publishing; Ratadelsistema.com.ar