Belorussian language
Belorussian, Ukrainian and Russian are a typical example of how one single language can be transformed in a different way being in different conditions. All three of them used to be one language named Old Russian. It was officially used and spoken in Kiev Russia, from the 9th until 13th century. Then Mongol conquest, Lithuanian and Polish expansion on Russian lands divided the nation, and new ethnic items appeared. Ukrainian and Belorussian suffered significant Polish influence, lost much of Russian original phonetics and some morphology and acquired a lot of Polish and Lithuanian vocabulary.

But still, Belorussian remains to be the closest tongue to Russian and represents a purely East Slavic language with all its flexions, the case system, and verb structure. Belorussian lost vocative case, lost dual number, aorist and perfect tenses. Three tenses that remain in the language are present, past and future. Two aspects, imperfective and perfective, verbal adverbs and participles, declinable adjectives and pronouns: all that is common with East Slavic languages.

As for phonetic system, Belorussian borrowed  from Polish deep sibilants and extensive use of [ts] sound instead of Russian soft [t']. The open [e] in the former Slavic berjoza (birch) is now pronounced as in Lithuanian or Polish like b'arjoza. The soft [d'] was converted to [dz] in Belorussian.