Novgorod dialect, although it 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 notable lexical similarities with Bulgarian due to a common Church Slavonic influence on both languages, but because of later interaction in the 19th and 20th centuries, Bulgarian grammar differs markedly from Russian. In the 19th century (in Russia until 1917), the language was often called "Great Russian" to distinguish it from Belarusian, then called "White Russian" and Ukrainian, then called "Little Russian" in the Russian Empire. 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, Italian, and English, and to a lesser extent the languages to the south and the east: Uralic, Turkic, Persian, Arabic, and Hebrew. 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 1,100 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 U.S. world policy. Standard Russian Feudal divisions and conflicts between rival polities created obstacles to the exchange of goods and ideas between the early medieval Rus' principalities before and especially during Mongol rule. This strengthened dialectal differences and for centuries prevented the establishment of any standardized "national" language. The gradual but steady emergence of the Grand Principality of Moscow (1263–1547) – later the Tsardom of Russia from 1547 – as the dominant and ever-expanding polity of the Rus', necessitated the earliest attempts at standardization of the East