Russian is a Slavic language in the Indo-European family. From the point of view of the spoken language, its closest relatives are Ukrainian and Belarusian, the other two national languages in the East Slavic group. In many places in eastern and southern Ukraine and throughout Belarus, these languages are spoken interchangeably, and in certain areas traditional bilingualism resulted in language mixtures, e.g. Surzhyk in eastern Ukraine and Trasianka in Belarus. An East Slavic Old Novgorod dialect, although vanished during the 15th or 16th century, is sometimes considered to have played a significant role in the formation of modern Russian. Also Russian has much in common with Bulgarian in vocabulary and phonetics as a result of interaction between the languages in the 19th–20th centuries, as well as the Church Slavonic influence on the both, although the Bulgarian grammar differs markedly from the Russian one.[20] In the 19th century, the language was often called Great Russian language based on the now discredited view that Russian, Ukrainian and Belarusian were dialects of a single language.
The vocabulary (mainly abstract and literary words), principles of word formations, and, to some extent, inflections and literary style of Russian have been also influenced by Church Slavonic, a developed and partly russified form of the South Slavic Old Church Slavonic language used by the Russian Orthodox Church. However, the East Slavic forms have tended to be used exclusively in the various dialects that are experiencing a rapid decline. In some cases, both the East Slavic and the Church Slavonic forms are in use, with many different meanings. For details, see Russian phonology and History of the Russian language.
Over the course of centuries, the vocabulary and literary style of Russian have also been influenced by Western and Central European languages such as Greek, Latin, Polish, Dutch, German, French, and English, and to a lesser extent the languages to the north and the east: Finno-Ugric, Turkic, Persian, Arabic.
According to the Defense Language Institute in Monterey, California, Russian is classified as a level III language in terms of learning difficulty for native English speakers, requiring approximately 780 hours of immersion instruction to achieve intermediate fluency. It is also regarded by the United States Intelligence Community as a "hard target" language, due to both its difficulty to master for English speakers and its critical role in American world policy.