Usage of 'the'
The Definite Article 'the' is used:
1) When we talk about a particular person or thing, or one already referred to.
E.g. 'The' book you want is out of print. (Which book? The one you want.)
2) When a singular noun is meant to represent a whole class.
E.g. 'The' dog loves comfort. (Here, the dog represents its whole breed.)
3) Before some proper names:
(a) Oceans and seas (the Pacific, the Black Sea)
(b) Rivers (the Ganga, the Nile)
(c) Canals (the Suez Canal)
(d) Deserts (the Sahara)
(e) Groups of islands (e.g. the West Indies)
(f) Mountain-ranges (the Himalayas, the Alps)
(g) A very few names of countries, which include words like republic and kingdom (the Irish Republic, the United Kingdom, also: the Ukraine, the Netherlands)
4) Before the names of certain books. E.g. The Vedas, the holy Quran, the Iliad.
But we say- Homer's Iliad.
5) Before names of things unique of their kind. E.g. The sun, the sky, the earth.