Short answers, pulled from the story.
The Balkans campaign began on the 28th of October 1940 with the Italian invasion of Greece and ended on the 1st of June 1941 when the last Allied forces on Crete surrendered. By that date, all of Albania, Yugoslavia, and Greece were under Axis control.
Hitler ordered the invasion of Greece, code-named Operation Marita, on the 13th of December 1940 primarily to prevent Britain from establishing air bases close enough to strike the Romanian oilfields. Germany also intervened to rescue Italy, whose invasion of Greece had stalled and been reversed by a Greek counter-offensive.
The Royal Yugoslav Army surrendered unconditionally on the 17th of April 1941, eleven days after the invasion began on the 6th of April. Germany and Italy dissolved the kingdom and created several successor entities, including the Independent State of Croatia (NDH), a German puppet government in Serbia led by Milan Nedic, Italian occupation of Montenegro, and Bulgarian annexation of most of modern-day North Macedonia.
The Battle of Crete began on the 20th of May 1941 when German paratroopers were dropped over the airfields of northern Crete. The remaining Allied forces surrendered on the 1st of June. It ranks as one of the largest airborne operations in the history of warfare, but German paratrooper losses were so severe that the Wehrmacht's Supreme Command abandoned large-scale airborne operations for the rest of the war.
Bulgaria had officially joined the Axis Powers but did not participate in the 6th of April 1941 invasions of Yugoslavia and Greece. On the 20th of April, the Bulgarian Army moved into Western Thrace and the Greek province of Eastern Macedonia, territory Germany had already conquered, aiming to restore Bulgaria's pre-World War I access to the Aegean Sea. Bulgaria also occupied much of eastern Serbia and held these territories for the remainder of the war.
On the 7th of April 1939, Italian troops invaded Albania, overthrew King Zog I, and annexed the country to the Italian Empire. This followed years of Italian interference in Albanian affairs under Mussolini dating from 1925, and it gave Italy the territory from which it launched its 1940 invasion of Greece.