Short answers, pulled from the story.
East Berlin became the capital of the German Democratic Republic in 1949, when the GDR was established. The Western Allied powers never formally recognized this claim, referring instead to East Berlin as the Soviet occupation sector and acknowledging it only as the "seat of government".
Construction of the Berlin Wall began on the 13th of August 1961, sealing East Berlin off from West Berlin. The Wall stood until the 9th of November 1989, separating the two halves of the city for nearly three decades.
East Berlin reached its highest recorded population of 1,284,535 on the 31st of December 1988, one year before the Berlin Wall fell. Its lowest point was in 1961, the year the Wall was built, at just over 1.055 million.
The solidarity surcharge, formally the Solidaritätszuschlaggesetz, was a 7.5 percent levy on personal and corporate income introduced in 1991-1992 to fund economic aid for the former East Germany after reunification. It was reintroduced in 1995 at 7.5 percent, then reduced to 5.5 percent in 1998, and continued to be levied beyond that date.
Eberhard Diepgen, a former mayor of West Berlin, became the first elected mayor of a reunified Berlin. He took office in January 1991, following citywide elections held in December 1990.
The Ampelmännchen is a stylized figure of a fedora-wearing man used on pedestrian traffic lights throughout the former East Berlin and the wider GDR. After reunification, a civic debate over whether to abolish or retain it ended with the symbol being adopted at some crosswalks in parts of the former West Berlin as well.