Use "the" if the noun is mentioned:
in an earlier sentence or paragraph
as part of an item already introduced
Use "the" if the noun is defined by:
a word (a proper noun) or determiner (this, that)
a prepositional or adjective phrase
a modifying clause
Use "the" if both listener and speaker:
know the one (the car we both own)
see the one (the book we are looking at)
culturally know the one (the post office)
or it is unique (the earth, the boy's mother)
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