Use 'the' when the noun is definite: unique, already known, or identifiable from context.
the + noun (often with a defining phrase/clause)
Don’t use 'the' for general plural/uncountable nouns: Dogs are friendly; Water is essential.
Can the listener answer: which one exactly? If yes, use 'the'.
💡 This rule is about definiteness (identifiability). It overlaps with first/second mention and shared knowledge, but it also covers context-defined nouns: the meeting we discussed, the answer to the question.
✓Please close the door.
✗Please close a door.
In the room, it is clear which door you mean.
✓The book on the table is mine.
✗A book on the table is mine. (if you mean that specific book)
A defining phrase makes it specific.
✓I watched a movie. The movie was great.
Second mention typically becomes definite.