Short answers, pulled from the story.
The Dnieper, Carpathian offensive began on the 24th of December 1943 and stretched until early May 1944. This operation involved almost 3,500,000 troops from both sides across a front line spanning 1,200 kilometers.
Economic motivations drove the need to control Ukraine's rich food resources and manganese production centers around Nikopol. Iron ore production centers in Krivoy Rog were also critical targets for the Red Army while Germany sought to export food and strategic raw materials to its homeland.
Nikolai Vatutin commanded the 1st Ukrainian Front while Ivan Konev led the 2nd Ukrainian Front. Rodion Malinovsky directed the 3rd Ukrainian Front and Fyodor Tolbukhin oversaw the 4th Ukrainian Front which conducted combined operations with the Separate Coastal Army and Black Sea Fleet.
Spring thaw created very muddy conditions that encumbered both armies during the winter of 1943-1944. Rivers became flooded and roads turned into rivers of mud often two feet deep making it difficult to drive any kind of vehicle except Russian T-34 tanks.
The Korsun, Shevchenkovsky offensive launched on the 24th of January 1944 resulted in encircling about 60,000 Germans in XI and XXXXII Army Corps around Korsun. Wilhelm Stemmermann commander of trapped forces decided to attempt final breakout on night of February 16-17 resulting in approximately 15,000 prisoners taken and at least 10,000 Germans killed including Stemmermann himself.