linguistic:voice_change, stylistic:tone, quality:correction
Basis

Written evidence of Old Church Slavonic survives in a relatively small body of manuscripts, most of them written in the First Bulgarian Empire during the late 10th and the early 11th centuries. The language has an Eastern South Slavic basis in the Bulgarian-Macedonian dialectal area, with an admixture of Western Slavic (Moravian) features inherited during the mission of Saints Cyril and Methodius to Great Moravia (863–885).

The only well-preserved manuscript of the Moravian recension, the Kiev Missal, or the Kiev Folia, is characterised by the replacement of some South Slavic phonetic and lexical features with Western Slavic ones. Manuscripts written in the Second Bulgarian Empire (1185–1396) have, on the other hand, few Western Slavic features.

Though South Slavic in phonology and morphology, Old Church Slavonic was influenced by Byzantine Greek in syntax and style, and is characterized by complex subordinate sentence structures and participial constructions.

A large body of complex, polymorphemic words was coined, first by Saint Cyril himself and then by his students at the academies in Great Moravia and the First Bulgarian Empire, to denote complex abstract and religious terms, e.g., ꙁълодѣꙗньѥ (zъlodějanьje) from ꙁъло ('evil') + дѣти ('do') + ньѥ (noun suffix), i.e., 'evil deed'. A significant part of them wеrе calqued directly from Greek.

Old Church Slavonic is valuable to historical linguists since it preserves archaic features believed to have once been common to all Slavic languages such as:

Old Church Slavonic is also likely to have preserved an extremely archaic type of accentuation (probably close to the Chakavian dialect of modern Serbo-Croatian), but unfortunately, no accent marks appear in the written manuscripts.

The South Slavic and Eastern South Slavic nature of the language is evident from the following variations:

Old Church Slavonic also shares the following phonetic features only with Bulgarian: