Complex sentence: In 2014, the Russian economy was the sixth largest in the world by PPP and tenth largest at market exchange rates, although the International Monetary Fund estimated that by the beginning of 2015 it dropped to nineteenth largest as result of significant depreciation of the ruble.
Split into simpler sentences: In 2014, the Russian economy was the sixth largest in the world by PPP and tenth largest at market exchange rates. However, the International Monetary Fund estimated that by the 2015 it could drop to nineteenth largest as result of depreciation of the ruble.