Short answers, pulled from the story.
Alexander Dubček was elected First Secretary of the Communist Party of Czechoslovakia on the 5th of January 1968. He subsequently launched an Action Programme of liberalizations in April that included increasing freedom of the press, speech, and movement.
Eastern Bloc armies from four Warsaw Pact countries invaded the country with 165,000 troops and 4,600 tanks entering on the night of the 20th to the 21st of August 1968. During this invasion 72 Czechs and Slovaks were killed while 266 were severely wounded and by the morning of the 21st of August Czechoslovakia was fully occupied.
Student Jan Palach set himself on fire in Prague's Wenceslas Square on the 16th of January 1969 to protest against renewed suppression of free speech. A memorial to the victims is located at Újezd at the bottom of Petrin hill featuring seven male bronze silhouettes descending a narrowing staircase.
Gustáv Husák replaced Alexander Dubček as first secretary in April 1969 and began a period of normalization that reversed all liberalizations. He dismissed professional and intellectual elites who openly expressed disagreement with the political transformation and instituted full censorship by March 1969 effectively ending hopes for freedoms.
An estimated 70,000 citizens fled the country immediately following the Soviet invasion while an eventual total reached some 300,000 people. The Soviet Press printed an unsigned request allegedly by Czechoslovak party leaders for immediate assistance including armed forces.