Article use in English – a/an, the or zero article
Those are the basic rules...
now some advanced points for definite and indefinite articles:
Classes (formal): The tiger is threatened with extinction.
To name a work by an artist: My mother found a Salvador Dali in the attic.
National groups: The Irish are known to be great writers and drinkers.
In measurements ‘a/an’ can be used instead of ‘per’: If petrol costs €1 a litre and your car is travelling at 50 miles an hour, how much will it cost to drive for 10 miles?
Some plural political/music/other groups: The government is evenly divided between the Social Democrats and the Green Party.
To name jobs: Otavio hopes to be a cosmonaut when he grows up.
Unique objects: Which is further from earth, the sun or the moon?
To emphasise a person is unknown: Miss Moneypenny said to Mr. Bond “a Mr. Evil called while you were saving the world”.
For titles (these tend to be unique): Anne Marie is the Director of Studies in our school.
Newspaper titles (‘The’ is capitalised): The Irish Times gives more accurate information than the Sun.
Use ‘the’ when talking about ability to play: Ko is learning how to play the bassoon.
Name of university only if post-modified: He studied in the University of Chicago, but his girlfriend studied in Oxford University.
Zero article before ‘most’ if it means ‘the majority’: Most Americans can’t point out Paris on a map.
‘The’ goes before most when used as a superlative: Paris is the most romantic city in Europe.
PRONUNCIATION OF THE ARTICLES
There are some things you need to remember about the pronunciation of articles. 'A' is used before consonant sounds and 'an' is used before vowel sounds. You need to listen to the sound, not just look at the spelling. So we say: 'An effect' and 'an FBI agent' because both start with the sound /e/. We say: 'A yellow submarine' and 'a European' because both start with /j/.
There are two ways to pronounce 'the'. 90% of the time we say: /ðə/. In some exceptional cases we pronounce 'the' as 'thee'( /ði:/ ).