The definite article "the" is used to refer to a specific noun that is already known to the reader or listener, or has been previously mentioned. It is used when both the speaker and the listener are aware of the particular thing or person being referred to. It can be used with singular and plural nouns, as well as with countable and uncountable nouns.
Example:
The cat is on the roof.(Refers to a specific cat that both the speaker and listener know.)
The books on the table are mine.(Refers to specific books that are already known to both.)
