The Bulgarian is at once the most ancient and the most modern of the languages which constitute the Slavonic group. In its groundwork it presents the nearest approach to the old ecclesiastical Slavonic, the liturgical language common to all the Orthodox Slavs, but it has undergone more important modifications than any of the sister dialects in the simplification of its grammatical forms; and the analytical character of its development may be compared with that of the neo-Latin and Germanic languages. The introduction of the definite article, which appears in the form of a suffix, and the almost total disappearance of the ancient declensions, for which the use of prepositions has been substituted, distinguish the Bulgarian from all the other members of the Slavonic family. Notwithstanding these changes, which give the language an essentially modern aspect, its close affinity with the ecclesiastical Slavonic, the oldest written dialect, is regarded as established by several eminent scholars, such as šafařik, Schleicher, Leskien and Brugman, and by many Russian philologists.

These authorities agree in describing the liturgical language as "Old Bulgarian." A different view, however, is maintained by Miklosich, Kopitar and some others, who regard it as "Old Slovene." According to the more generally accepted theory, the dialect spoken by the Bulgarian population in the neighbourhood of Salonica, the birthplace of SS. Cyril and Methodius, was employed by the Slavonic apostles in their translations from the Greek, which formed the model for subsequent ecclesiastical literature. This view receives support from the fact that the two nasal vowels of the Church-Slavonic (the greater and lesser ûs), which have been modified in all the cognate languages except Polish, retain their original pronunciation locally in the neighbourhood of Salonica and Castoria; in modern literary Bulgarian the rhinesmus has disappeared, but the old nasal vowels preserve a peculiar pronunciation, the greater ûs changing to ŭ, as in English "but," the lesser to ĕ, as in "bet," while in Servian, Russian and Slovene the greater ûs becomes ū or ō, the lesser e or ya.

The remnants of the declensions still existing in Bulgarian (mainly in pronominal and adverbial forms) show a close analogy to those of the old ecclesiastical language.

The Slavonic apostles wrote in the 9th century (St Cyril died in 869, St Methodius in 885), but the original manuscripts have not been preserved. The oldest existing copies, which date from the 10th century, already betray the influence of the contemporary vernacular speech, but as the alterations introduced by the copyists are neither constant nor regular, it is possible to reconstruct the original language with tolerable certainty. The "Old Bulgarian," or archaic Slavonic, was an inflexional language of the synthetic type, containing few foreign elements in its vocabulary. The Christian terminology was, of course, mainly Greek; the Latin or German words which occasionally occur were derived from Moravia and Pannonia, where the two saints pursued their missionary labours. In course of time it underwent considerable modifications, both phonetic and structural, in the various Slavonic countries in which it became the liturgical language, and the various MSS. are consequently classified as "Servian-Slavonic," "Croatian-Slavonic," "Russian-Slavonic," etc., according to the different recensions.

The "Russian-Slavonic" is the liturgical language now in general use among the Orthodox Slavs of the Balkan Peninsula owing to the great number of ecclesiastical books introduced from Russia in the 17th and 18th centuries; until comparatively recent times it was believed to be the genuine language of the Slavonic apostles. Among the Bulgarians the spoken language of the 9th century underwent important changes during the next three hundred years. The influence of these changes gradually asserts itself in the written language; in the period extending from the 12th to the 15th century the writers still endeavoured to follow the archaic model, but it is evident that the vernacular had already become widely different from the speech of SS. Cyril and Methodius. The language of the MSS. of this period is known as the "Middle Bulgarian"; it stands midway between the old ecclesiastical Slavonic and the modern speech.

In the first half of the 16th century the characteristic features of the modern language became apparent in the literary monuments. These features undoubtedly displayed themselves at a much earlier period in the oral speech; but the progress of their development has not yet been completely investigated. Much light may be thrown on this subject by the examination of many hitherto little-known manuscripts and by the scientific study of the folk-songs. In addition to the employment of the article, the loss of the noun-declensions, and the modification of the nasal vowels above alluded to, the disappearance in pronunciation of the final vowels yer-golêm and yer-malúk, the loss of the infinitive, and the increased variety of the conjugations, distinguish the modern from the ancient language. The suffix-article, which is derived from the demonstrative pronoun, is a feature peculiar to the Bulgarian among Slavonic and to the Rumanian among Latin languages. This and other points of resemblance between these remotely related members of the Indo-European group are shared by the Albanian, probably the representative of the old Illyrian language, and have consequently been attributed to the influence of the aboriginal speech of the Peninsula. A demonstrative suffix, however, is sometimes found in Russian and Polish, and traces of the article in an embryonic state occur in the "Old Bulgarian" MSS. of the 10th and 11th centuries.

In some Bulgarian dialects it assumes different forms according to the proximity or remoteness of the object mentioned. Thus zhena-ta is "the woman"; zhena-va or zhena-sa, "the woman close by"; zhena-na, "the woman yonder." In the borderland between the Servian and Bulgarian nationalities the local use of the article supplies the means of drawing an ethnological frontier; it is nowhere more marked than in the immediate neighbourhood of the Servian population, as, for instance, at Dibra and Prilep. The modern Bulgarian has admitted many foreign elements. It contains about 2000 Turkish and 1000 Greek words dispersed in the various dialects; some Persian and Arabic words have entered through the Turkish medium, and a few Rumanian and Albanian words are found. Most of these are rejected by the purism of the literary language, which, however, has been compelled to borrow the phraseology of modern civilization from the Russian, French and other European languages. The dialects spoken in the kingdom may be classed in two groups - the eastern and the western.

The main point of difference is the pronunciation of the letter yedvoïno, which in the eastern has frequently the sound of ya, in the western invariably that of e in "pet." The literary language began in the western dialect under the twofold influence of Servian literature and the Church Slavonic. In a short time, however, the eastern dialect prevailed, and the influence of Russian literature became predominant. An anti-Russian reaction was initiated by Borgoroff (1818-1892), and has been maintained by numerous writers educated in the German and Austrian universities. Since the foundation of the university of Sofia the literary language has taken a middle course between the ultra-Russian models of the past generation and the dialectic Bulgarian. Little uniformity, however, has yet been attained in regard to diction, orthography or pronunciation.

The Bulgarians of pagan times are stated by the monk Khrabr, a contemporary of Tsar Simeon, to have employed a peculiar writing, of which inscriptions recently found near Kaspitchan may possibly be specimens. The earliest manuscripts of the "Old Bulgarian" are written in one or other of the two alphabets known as the glagolitic and Cyrillic (see Slavs). The former was used by Bulgarian writers concurrently with the Cyrillic down to the 12th century. Among the orthodox Slavs the Cyrillic finally superseded the glagolitic; as modified by Peter the Great it became the Russian alphabet, which, with the revival of literature, was introduced into Servia and Bulgaria. Some Russian letters which are superfluous in Bulgarian have been abandoned by the native writers, and a few characters have been restored from the ancient alphabet.