The: it is a definite article that introduces a noun phrase.
It's use is to specify general words in to specific words. Example: 1.Can you reach me "a" book.
2.Can you reach me "the" book.
In the first sentence it is referred to any book whereas in the second phrase it is referred to a specific book.
You should use the if you are talking abut something that you have already mentioned it. Example: She has got two dogs: a female and a male; the female is eight years old.
If you want to say you play a music instrument you always have to use "the". It happens the same with countries, mountains, rivers, seas, oceans and geographic areas. Example: The Nile is the longest river.
It is used for unique things too. Example: The moon, The sun...