1. A singular or plural noun when it is clear/obvious which person or thing we are talking about.
There is a lamp in my bedroom. (we mention the lamp for the first time)
The lamp is next to the desk.
2. Anything which we identify immediately.
We watched the new Brad Pitt movie last night.
3. Musical instruments (the violin, the guitar, the drums, the flute, the piccolo).
She plays the piano.
4. Something that is unique or there is only one.
the sun
the moon
the internet.
5. Names of rivers, seas, oceans, mountain ranges and deserts (always in capitals).
The Mississippi River
The Black Sea
The Andes
The Sahara Desert
6. Directions (cardinal points).
the west
the south-east
the north-west.