We use "the" when we are talking about a specific,definite object. "THe computer which I bought is very good". "The people of Italy are very friendly". "The boy put his bag on the table" - it means we know which boy we are referring to. https://learnenglish.britishcouncil.org/en/english-grammar/determiners-and-quantifiers/definite-article

'The' is the definitive article (l'articolo definitivo) and we use it in the same way as you would use la, le, il, i etc.although we only have the one word for 'the' unlike Italian.

We use "the" when both you (reader/listener) and I (writer/speaker) know exactly which one is meant. This could mean it's been mentioned before, or there's only one, or it's obviously a certain one as opposed to others, or (in Ruthi's first example about the computer) we immediately explain which one we mean.

You should also know that around 15% of all article use is articles in fixed expressions and idioms and there are no rules for them. For example, we say 'go THE extra mile' or 'have lunch' but if you add an adjective you'll have to use an indefinite article 'we had a nice lunch'. This link can also be helpful: http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv198.shtml

i agree with Ruthi & Paul. one more thing that i remember is that when you want to give some value to anything you also can use "The". for example The city (city name). On the day. In the news. and so on.