1. Function
1.1. The definite article the is the most frequent word in English. We use the definite article in front of a noun when we believe the listener/reader knows exactly what we are referring to:
1.1.1. Because there is only one
1.1.2. Because there is only one in that context
1.1.3. Because we have already mentioned it
1.1.4. To say something about all the things referred to by a noun
1.1.5. To refer to a system or service
2. Use
2.1. With
2.1.1. Ordinal numbers
2.1.2. Countries or places where the name refers to a group of islands or states
2.1.3. Countries that have plural names
2.1.4. Superlatives
2.1.5. Decades
2.1.6. When there is only one in the world
2.1.7. To refer to the only one in this particular area
2.1.8. Adjectives, to refer to a whole group of people
2.1.9. Names of geographical areas
2.1.10. Musical instrument
3. Incorrect
3.1. Use
3.1.1. Before names of people, streets, islands, mountains, cities, countries and continents.
3.1.2. Before names of days and months
3.1.3. Before plural countable nouns or uncountable nouns
3.1.4. Before abstract nouns
3.1.5. Before names of magazine, sports, games, colours and languages.