History of the Bulgarian language

The History of the Bulgarian language can be divided into three major periods:

Bulgarian as a written South Slavic language that dates back to the end of the 9th century.

Old Bulgarian was the first literary period in the development of the language. It was a highly synthetic language with a rich declension system as attested by a number of manuscripts from the late 10th and the early 11th centuries. Those originate mostly from the Preslav and the Ohrid Literary School, although smaller literary centers also contributed to the tradition. The language became a medium for rich scholarly activity — chiefly in the late 9th and the early 10th century — with writers such as Constantine of Preslav, John Exarch, Clement of Ohrid, Chernorizetz Hrabar and Naum of Preslav (Naum of Ohrid). Most of their works are preserved through later copies, many of which are from neighboring Balkan countries or Russia.

The name “Old Bulgarian” was extensively used in the late 19th and the first half of the 20th century synonymously with Old Church Slavonic to describe the literary language of a number of Slavic peoples from the 9th until the 12th century. Although "Old Bulgarian" is still used in a number of sources with the meaning "Old Church Slavonic", there is a growing tendency for the name to be applied only to the language of manuscripts from the First Bulgarian Empire (Bulgarian editions of Old Church Slavonic), excluding manuscripts from other editions.

Old Bulgarian is characterized by a number of phonetic, morphological, syntactic, and lexical traits (some of which are shared with other Slavic languages and some, such as the reflexes of *tj ([t']) and *dj ([d']), are typical only for Bulgarian):

In the period between 12th and 15th centuries the structure of the language changed quite radically. Few of these changes can still be observed in contemporary written records, thanks to the tendency towards archaicism driven by a desire to preserve the purity of the Cyrilo-Methodian tradition.

...
Wikipedia