Short answers, pulled from the story.
Revolutionaries under Sun Yat-sen proclaimed the Republic of China on the 1st of January 1912 in Nanjing. This event marked the end of over two thousand years of imperial rule following the Xinhai Revolution.
Puyi was the final emperor of the Qing dynasty who abdicated on the 12th of February 1912. His abdication followed decades of instability and foreign pressure that caused the collapse of the Qing dynasty.
Popular unrest forced Yuan Shikai to abandon his plan to establish an imperial dynasty before he died in 1916. His death triggered a period of decentralized conflicts between rival military cliques known as warlord states.
Chiang Kai-shek unified the entire nation nominally by 1928 through the Northern Expedition which began the Nanjing decade. He established a government with Nanjing as its capital in 1927 after pushing the Communists into the interior.
Open warfare began after the 7th of July 1937 when clashes occurred near the Marco Polo Bridge outside Beijing. This conflict led to the fall of Shanghai and later the mass murders known as the Nanjing Massacre in December 1937.
The demoralized National Revolutionary Army proved no match for the Communists' motivated People's Liberation Army despite advantages in numbers and territory. On the 23rd of April 1949 major cities passed from Kuomintang to Communist control with minimal resistance leading to the retreat to Taiwan.