In the earliest phase, Ukrainian underwent the same development as other Slavic languages – after the migration of the Slavs in Eastern Europe in the sixth century AD, three main groups of Slavic languages emerged by the tenth century: Western, Southern and Eastern. Ukrainian, along with Belarusian and Russian, descend from the latter group.
There are several theories about the later development of Ukrainian. One theory suggests it was because of foreign influence, others tell a more radical story: that the three East Slavic languages existed as distinct languages after their separation from the Proto-Slavic language.
The most widely accepted theory by linguists outside Ukraine says all three languages developed their distinct characteristics naturally. Until the 12th or 13th century, Ukrainian, Russian and Belarusian were almost indistinguishable. This was during the Kievan Rus state period. After its fall, Ukrainian started to move further away from Russian. In the 14th century, the south-western territories of modern Ukraine came under the power of the Grand Duchy of Lithuania. The eastern part, in turn, was under the rule of the Tatars and was later called the Tsardom of Muscovy. So the language of the two regions evolved in isolation from each other for the next four centuries.
A feature of rule of the Grand Duchy of Lithuania was that it allowed wide autonomy of the state, language and culture. In the Grand Duchy of Lithuania, Old Slavic became the chancellery language and gradually evolved into Ruthenian.
After the Union of Lublin in the 16th century that formed the Polish-Lithuanian Commonwealth, the territory of Ukraine fell under Polish administration. This was marked by attempts at assimilation and Polonisation. The Ukrainian upper class learned Polish, while the lower classes were less affected despite significant pressure and due to their low literacy rate.
From the mid-17th century, part of Ukraine’s territory came under the domination of the Russian Empire. And from the 18th century, the eastern part of Ukraine’s territory was incorporated into the Russian Empire.