Short answers, pulled from the story.
The Mongolian People's Republic was founded in 1924, following the Mongolian Revolution of 1921, and formally ended in February 1992 when a new constitution creating a unicameral State Great Khural came into force. It existed for nearly 68 years as a communist state.
Between 20,000 and 35,000 Mongolians were executed in Mongolia and the Soviet Union during the purges of September 1937 to April 1939. An estimated 17,000 monks were killed, and Buddhist institutions were nearly all destroyed.
Khorloogiin Choibalsan dominated Mongolia from March 1939 until his death in Moscow in 1952. Yumjaagiin Tsedenbal then ruled in various capacities until he was removed by the Politburo in August 1984, citing "old age and mental incapacity".
The Battle of Khalkhin Gol was a series of clashes in 1939 between Japanese forces and Soviet-Mongolian troops on Mongolia's eastern border. Soviet and Mongolian forces under General Georgy Zhukov encircled and destroyed the Japanese units in August 1939, ending Japan's expansionary push toward Mongolia.
Protests began on the 10th of December 1989, when young people demonstrated for political freedom in Ulaanbaatar. The MPRP leadership resigned in March 1990, opposition parties were legalized in May, and the first multiparty elections were held in July. A new constitution adopted in January 1992 formally abolished the communist state.
The Soviet Union was Mongolia's dominant economic partner throughout the republic's existence, providing credits, advisers, and joint ventures through the Comecon framework. By 1990, long-term Soviet loans to Mongolia were estimated at 10 billion rubles, and roughly 85 percent of Mongolia's trade remained with the USSR.