Short answers, pulled from the story.
The occupation of Japan began on the 2nd of September 1945, the date Japan formally surrendered, and ended on the 28th of April 1952, when the Treaty of San Francisco took effect and full sovereignty was restored to Japan.
General Douglas MacArthur was appointed Supreme Commander for the Allied Powers by President Harry S. Truman. MacArthur held this position until 1951, when he was succeeded by General Matthew Ridgway.
Emperor Hirohito was granted full immunity from prosecution for war crimes by General MacArthur. Historian John W. Dower wrote that MacArthur's headquarters effectively functioned as a defense team for the emperor, working to ensure Hirohito was not indicted by the Tokyo tribunal.
Japan's 1947 Constitution was drafted by American civilian officials within SCAP and ratified by the Diet as an amendment to the Meiji Constitution. It transferred sovereignty from the Emperor to the people, included Article Nine renouncing war, and enfranchised women. The articles guaranteeing equality between men and women were written by American civilian Beate Sirota.
The Reverse Course was a major shift in occupation policy that began in 1947, in response to the emerging Cold War. US priorities shifted from punishing and reforming Japan to stabilizing the Japanese economy, suppressing left-wing forces, and preparing Japan as a Cold War ally in East Asia, including reversing some earlier reforms.
Nearly one million American soldiers served in the occupation over its seven-year duration, including rotations of replacement troops. By the end of 1945, around 430,000 American soldiers were stationed throughout Japan. The British Commonwealth Occupation Force added around 40,000 personnel at its peak.