This is the 8th most frequent Danish word.
den
"The" (definite article) or "it" (pronoun), depending on context.
Here, 'den' is used as a pronoun to refer back to a previously mentioned or understood object, equivalent to 'it' in English.
Jeg lånte den i går.
I borrowed it yesterday.
In this sentence, 'den' functions as a definite article for the noun 'bog' (book), meaning 'the', and matches the gender and number of the noun.
Den gamle bog på bordet er min.
The old book on the table is mine.
In this usage, 'den' is a demonstrative pronoun referring specifically to 'den avis' (the newspaper), implying 'that' in translation.