Short answers, pulled from the story.
Bohdan Khmelnytsky was a Ruthenian nobleman and Hetman of the Zaporozhian Host who led a Cossack uprising against the Polish-Lithuanian Commonwealth from 1648 to 1654. He founded an independent Cossack state in Ukraine and concluded the 1654 Treaty of Pereiaslav, placing Ukraine under Russian protection. His uprising caused the deaths of hundreds of thousands of people, including an estimated 18,000-100,000 Jews.
The Treaty of Pereiaslav was concluded on the 18th of January 1654 between Khmelnytsky's Cossack Hetmanate and Tsar Alexei Mikhailovich of Russia. Historians have not reached a consensus on whether it constituted a military alliance, a suzerainty, or a full incorporation of Ukraine into the Tsardom of Russia. The treaty legitimized Russian claims to the capital of Kievan Rus and placed Ukraine under Russian protection.
Estimates of Jewish deaths during the Khmelnytsky Uprising range from 18,000 to 100,000, including deaths from starvation and disease. Between 1648 and 1656, tens of thousands of Jews were killed by Cossack rebels, and survivors were among those sold into slavery to the Crimean Tatars. The uprising is considered one of the most traumatic events in Jewish history.
Khmelnytsky was probably born in the village of Subotiv, near Chyhyryn, around 1595. He died at 5 a.m. on the 27th of July 1657 after suffering a cerebral hemorrhage on the 22nd of July. His body was taken from his capital at Chyhyryn to Subotiv for burial in his ancestral church.
In modern Ukraine, Khmelnytsky is generally regarded as a national hero. A 2018 poll by Ukraine's Rating Sociological Group found that 73% of Ukrainian respondents had a positive attitude toward him. He appears on Ukrainian banknotes, a city and region bear his name, and his monument stands at the centre of Kyiv. However, he is also criticized for his alliance with Russia, his cooperation with the Crimean Tatars, and the enslavement of Ukrainian peasants those alliances enabled.
The Czaplinski Affair refers to the seizure of Khmelnytsky's Subotiv estate by Daniel Czaplinski, acting under authority from magnate Aleksander Koniecpolski. In April 1647, Czaplinski evicted Khmelnytsky from the land, had his son Yuriy badly beaten, and abducted and forcibly married the woman Khmelnytsky planned to wed. Khmelnytsky's failure to obtain relief from the Polish king or courts drove him to the Zaporozhian Sich, where he was elected hetman in January 1648 and launched the uprising.