(ARTICLES I) We use "the" when....

...there is only one, we know what is being talked about, and with superlatives, musical instruments, oceans, mountain ranges, deserts, rivers, and some countries.
And;

When there is only one of something, either in existence or in a particular context.

when something is mentioned again

when both listener and speaker know what is being refered to

when talking about a specific aspect of something, where the noun is followed by "of"

to speak generally about certain groups of singular countable nouns.