Why is the emperor before the Tang Dynasty called Emperor X, and the emperor after the Tang Dynasty called School X?

Before the Tang Dynasty, people used to call the former emperors mainly posthumous title, so they were all called Emperor XX. Since Emperor Taizong, some people who used to curry favor with posthumous title felt disrespectful, and simply called it Dian Hao. Since then, except for Mao, most of them have been called Emperor XX, and all of them are kings of subjugation. Because there is no temple name, people can only call him posthumous title or Nianhao, such as,, Wendi. As for calling an emperor by his title, it is a folk title. Because it is concise and clear, it has been used for reference by later scholars. The title of emperor originated in the Yuan Dynasty for two reasons. First, the Han culture learned to resist the rulers of the Yuan Dynasty. They don't agree with these foreign rulers at all, and certainly don't regard them as ancestors, so they use the year number as an agent. The other is from the imperial calendar. Due to the destruction of Han culture by the rulers of Yuan Dynasty, the illiteracy rate of ordinary people is extremely high. However, for their own needs, the rulers of the Yuan Dynasty promoted the "imperial calendar". When the Yuan Dynasty changed, they had to re-publish the new yellow calendar with the emperor's year number printed on it. People who can't read much use the year number to refer to the emperor, and gradually people get used to it.