Okay, kiddo, so a long time ago in China, there was a government called the Republic of China. Some really smart people tried to understand how the government changed over time, and they came up with a theory called the four-stage theory.
The first stage was called the Revolutionary era, which happened from 1911 to 1927. This was when the Republic of China was first established, and they were trying to get rid of the old government that had been in charge for a very long time. They were fighting really hard to make things better for the people.
The second stage was called the Warlord era, which happened from 1927 to 1937. This was a pretty chaotic time. There were lots of different groups competing for power in China, and they were all fighting against each other. The government was weak and couldn't really control what was happening.
The third stage was called the Nanjing decade, which happened from 1937 to 1949. This was a time when the Republic of China finally had some stability. They had a strong leader, Chiang Kai-shek, who was able to create some order and make things better for the people.
The fourth and final stage was called the Communist era, which happened from 1949 onwards. This was when a new government took over, led by the Communist Party. They believed in different things than the Republic of China, and they wanted to make changes to the country in a different way.
So, there you have it, kiddo - the four-stage theory of the Republic of China! It's a way of understanding how the government changed over time, from when it was first established to when a new government took over.