With the union of the crowns of Castille and Aragon (1479), the use of Spanish gradually became more prestigious. Starting in the 16th century, Catalan literature experienced a decline, the language came under the influence of Spanish, and the urban and literary classes became bilingual.
Is there an answer to this question (If it cannot be answered, say "unanswerable"): When did Catalan literature begin a downward trend?
16th century