Short answers, pulled from the story.
The Yalta Conference opened on the 4th of February 1945 inside the Livadia Palace near the Black Sea resort town of Yalta in Crimea. The event took place within three palaces including the Yusupov and Vorontsov estates during eight days of intense negotiation.
Roosevelt sought Soviet support for the Pacific War against Japan to reduce American casualties while Churchill pressed for free elections in Central and Eastern Europe with a focus on Poland. Stalin demanded a Soviet sphere of political influence in Eastern and Central Europe as an essential aspect of national security strategy.
The Soviets kept the territory of eastern Poland that they had annexed in 1939 while Poland received compensation by extending its western borders at the expense of Germany. The Polish eastern border followed the Curzon Line established after World War I and the final agreement stipulated that the provisional government should be reorganized on a broader democratic basis.
Stalin requested that all 16 Soviet Socialist Republics would be granted UN membership because he wanted voting power aligned with his republics. Roosevelt and Churchill agreed to membership for Ukraine and Byelorussia while denying the other 14 republics despite initial discussions about additional votes.
Poland's official transformation to a communist state occurred by 1949 after the Polish elections held on the 16th of January 1947 resulted in the country becoming a communist state. Many Poles felt betrayed by their wartime allies and many Polish soldiers refused to return to Poland due to Soviet repressions of citizens between 1939 and 1946.