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