1. to refer to something which was mentioned before:
A horse and a donkey loved each other. The horse loved the donkey's beautiful eyes, and the donkey loved the horse's long hair.
2. The speaker and listener know what is being talked about, even if it wasn’t mentioned before.
The boy is listening to music.
3. In sentences or clauses where we define or identify a particular person or object:
Which bag is yours? The red one.
4. to refer to objects we regard as unique.
The earth is going around the sun.
5. with musical instrument.
I can play the guitar.
6. before superlatives and ordinal numbers.
Burj Khalifa is the tallest building in the world.
The first chapter of the book is very interesting.
7. with adjectives, to refer to a whole group of people:
the Japanese, the old
8. with names of geographical areas and oceans:
the Caribbean, the Sahara, the Atlantic
9. with decades, or groups of years:
She grew up in the seventies.