How did japan's role in wwii affect north and south korea?

 Korea had been under Japanese domination since 1910, and had been very isolationist for most of the centuries before then. During the Japanese occupation, Japan tried to eliminate Korea as a separate country - suppressing their local government and their language, forcing Koreans to take Japanese names, destroying or stealing hundreds of cultural artifacts. When Japan surrendered in 1945 it was forced to give up conquered territories, including Korea. 

Hundreds of thousands of Koreans died during WWII, as forced laborers and conscripted soldiers. Perhaps as many as 100,000 Korean girls and women were forced into sexual slavery as "comfort women". By the end of the war, more than 800,000 Japanese colonists were occupying Korea, though the majority of those returned to Japan after the surrender. 

When Soviet Russia entered the war against Japan in the few days before Japan surrendered, it claimed the right to occupy some Japanese territories during the transition period to what was supposed to be home rule for former Japanese conquests. Between 1945 and 1948, while the United Nations was trying to help the Koreans set up an independent government, the Soviet Russians were working hard in their occupation zone to set up a communist government in their own image - this led to two Koreas, North and South, which led to the Korean War and the current stalemate between the two Koreas. The partition of Korea into North and South is a direct result of the end of World War II.