See number 4.
Articles in English are invariable. That is, they do not change according to the gender or number of the noun they refer to, e.g. the boy, the woman, the children
1. to refer to something which has already been mentioned.
Example: An elephant and a mouse fell in love.The mouselovedthe elephant'slong trunk,andthe elephantlovedthe mouse'stiny nose.2. when both the speaker and listener know what is being talked about, even if it has not been mentioned before.
Example: 'Where's the bathroom?'3. in sentences or clauses where we define or identify a particular person or object:
'It's on the first floor.'
Examples: The man who wrote this book is famous.4. to refer to objects we regard as unique:
'Which car did you scratch?' 'The red one.
My house is the one with a blue door.'
Examples: the sun, the moon, the world 5. before superlatives and ordinal numbers: (see Adjectives)
Examples: the highest building, the first page, the last chapter. 6. with adjectives, to refer to a whole group of people:7. with names of geographical areas and oceans:
Examples: the Caribbean, the Sahara, the Atlantic 8. with decades, or groups of years:
Example: she grew up in the seventies