This thesis is devoted to the analysis of a little-known group of Bulgarian indefinite pronouns with the particle si (някой си, някакъв си) and their English Parallels identification. The research is based on material taken from the Bulgarian national corpus and the parallel corpus of Slavic languages ParaSol. The study examines semantic, grammatical, referential and contextual features of this type of indefinite pronouns, describes the contexts that provoke the manifestation of a pragmatic and evaluative component in their semantics. The relationship between the semantics of an indefinite pronoun, the reference of a nominal expression, and the structure of a nominal group with one or another indefinite pronoun is determined. The study of the material of the parallel corpus showed that the semantics of the Bulgarian pronouns някой си and някакъв си is transmitted in English not only and not mainly by indefinite pronouns, but by a wide range of means related to the sphere of uncertainty.