Bulgarian is closely related to Macedonian, generally recognized as a distinct language, although the prevalent opinion in Bulgaria, to some extent in Greece, and that of certain international linguists is that Bulgarian and Macedonian are two standard forms of the same diasystem.
Bulgarian and Macedonian form part of the Balkan linguistic union, which also includes Greek, Romanian, Albanian and some Serbian dialects. Most of these languages share some of the above-mentioned characteristics (e.g., definite article, infinitive loss, complicated verb system) and many more. The "nonwitnessed action" verb forms, pertaining to a mood known as renarrative mood, have been attributed to Turkish influences by most Bulgarian linguists. Morphohologically, they are related to the perfect tenses, which are known in Bulgarian linguistic tradition as "preliminary" tenses.
The development of the Bulgarian language may be divided into several historical periods. The prehistoric period (essentially proto-Slavic) occurred between the Slavonic invasion of the eastern Balkans and the mission of St. Cyril and St. Methodius to Great Moravia in the 860s. Old Bulgarian (9th to 11th century, also referred to as Old Church Slavonic) was the language used by St. Cyril, St. Methodius and their disciples to translate the Bible and other liturgical literature from Greek. Middle Bulgarian (12th to 15th century) was a language of rich literary activity and major innovations. Modern Bulgarian dates from the 16th century onwards; the present-day written language was standardized on the basis of the 19th-century Bulgarian vernacular. The historical development of the Bulgarian language can be described as a transition from a highly synthetic language (Old Bulgarian) to a typical analytic language (Modern Bulgarian) with Middle Bulgarian as a midpoint in this transition.
Fewer than 20 words remain in Bulgarian from the language of the Bulgars, the Central Asian people who moved into present-day Bulgaria and eventually adopted the local Slavic language. The Bolgar language, a member of the Turkic language family or the Iranian language family (Pamir languages), is otherwise unrelated to Bulgarian.
In 886 AD, Bulgaria adopted the Glagolitic alphabet which was devised by the Byzantine missionaries Saint Cyril and Methodius in the 850s. The Glagolitic alphabet was gradually superseded in later centuries by the Cyrillic alphabet, developed around the Preslav Literary School in the beginning of the 10th century. Most letters in the Cyrillic alphabet were borrowed from the Greek alphabet, but those which had no Greek equivalents represent simplified Glagolitic letters.
Under the influence of printed books from Russia, the Russian "civil script" of Peter I (see Reforms of Russian orthography) replaced the old Middle Bulgarian/Church Slavonic script at the end of the 18th century. Several Cyrillic alphabets with 28 to 44 letters were used in the beginning and the middle of the 19th century during the efforts on the codification of Modern Bulgarian until an alphabet with 32 letters, proposed by Marin Drinov, gained prominence in the 1870s. The present Bulgarian alphabet has 30 letters.