Short answers, pulled from the story.
Soviet forces invaded Finland on the 30th of November 1939. This attack occurred three months after the outbreak of World War II and ended with the Moscow Peace Treaty on the 13th of March 1940.
The Soviets demanded that Finland cede or lease islands in the Gulf of Finland along seaward approaches to Leningrad. They also proposed moving the Finnish-Soviet border westward to a point east of Viipuri while demanding the cession of Rybachy Peninsula and a 30-year lease of the Hanko Peninsula for a military base.
Stalin's purges resulted in the loss of 36,761 officer ranks including three of five marshals and 220 division or higher-level commanders. Fewer than half all officers remained because they were commonly replaced by soldiers less competent but more loyal to superiors.
The Battle of Raate Road resulted in one of the largest Soviet losses of the entire conflict. The Soviet 44th parts 163rd Rifle Division comprising about 14,000 troops was almost completely destroyed by Finnish ambush tactics.
Hostilities ceased with Finland ceding 9 percent territory to the Soviet Union under the Moscow Peace Treaty. Despite heavy Soviet losses and international reputation damage, the Soviets received substantial territories along Lake Ladoga further north that exceeded their pre-war demands.