To understand this, you need to go back to the 13th century. At that time there was Kievan Rus, a group of duchies in what is now ukraine, Belarus and western Russia.
They battled against the Khazars, byzantines, poles, bulgarians etc. but they were destroyed by the Mongol invasion
THAT was the start of the separation of "Russians" (more properly called Muscowites), Belarusyni and Ruthenians (Ukrainians).
Moscow in the 13th century was an outpost of the state of Vladimir-suzdal and got it's prominence by agreeing to be the tax-collector for the Great Khan.
As the Mongol power waned, the Lithianian Grand Dukes (pagans) came to power and took over western Ukraine and Belarus.
Then in 1410 they joined with the POles to form the Polish-Lithuanian commonwealth (a multi-national state that was larger than the Holy Roman Empire at that time).
As time went by the BElarusyni were under Lithianian 'control' but really ruled themselves while the Ukrainians merged with the Poles -- the upper classes became polish while the lower stayed Ruthenian (the languages are roughly compatible, roughly like Flemish and English)
Moscow on the other hand grew in power as a tax collector and then 100 years later threw off the Mongol yoke. ARguably they absorbed a lot of the Mongol system of governance while the Ruthenians/Ukrainians absorbed Poland's chaotic democracy.
The Muscowites then, after the fall of Constantinople, decided that they were not only the "third Rome" but they also had the divine task of "the gathering of the Rus" -- gathering all the Eastern Slavs (and then all the Eastern Orthodox) under Caesar/Tsar.
In the 1700s the Cossacks in what is now Ukraine revolted against the Poles but jumped out of the Polish sizzling pan into the Russian fire. They wanted to keep their culture and language intact but Polish culture and langauge was too advanced and culturally compelling. Russian on the other hand was not so advanced but had a state that didn't care about human life.
and that was Ukraine's faith for 200 years, slowly Russified
The "Russians" in the Ukraine are those who speak the Russian language while Ukrainians are mainly just based on language.
Ukraine never had a chance to build up a higher culture as it was surrounded by more advanced cultures...
ah! so Ukrainians are stupid, a bit backward wouldn't you say? Makes sense though since other cultures stole from them.