How to do this historical problem: What is Yan Zhenqing's achievement in calligraphy?

Yan Zhenqing founded "Yan Ti" regular script, and was called "four masters of regular script" with Zhao Mengfu, Liu Gongquan and Ou Yangxun. And Liu Gongquan are also called "Ada". Yan Zhenqing is a descendant of the Yan family of Langya, with a deep family background. Yan Zhitui, the sixth ancestor, was a famous scholar in the Northern Qi Dynasty. He once wrote family instructions for Yan. When Yan Zhenqing was young, his family was poor and lacked paper and pens, so he used a pen to brush loess water to practice calligraphy on the wall. His calligraphy style is called "Yan Ti", which is also called "Yan Liu" with Liu Gongquan, and has the reputation of "Yan Liu Jin Gu". And Yan Zhenqing's calligraphy pen is fat and stupid, which makes him feel uncomfortable. The "flying white book" of cutting-edge calligraphy art is not obvious in Yan's family; In the Eastern Jin Dynasty, Mrs. Wei wrote "Bi Tan": "Those who are bony and fleshy are called, and those who are fleshy are called". However, although Yan's calligraphy is full of dignity and respect, it is definitely not the so-called "Mo Zhu". His regular script is full of bones and muscles, vigorous, powerful and magnificent, which has set a new trend for a generation. If there is anyone in China who can compete with Wang Xizhi, it is only Yan Zhenqing who has this prestige and influence. His running script is vigorous and powerful. Apollo's style also embodies the prosperous style of the Tang Dynasty Empire, which is consistent with his noble personality. It is a typical example of the perfect combination of calligraphy beauty and personality beauty, so it is praised as "the second running script in the world" by later generations. His calligraphy style is called "Yan Ti", which is also called "Yan Liu" with Liu Gongquan, and has the reputation of "Yan Liu Jin Gu". Yan Zhenqing's brushwork is even and hidden, just inside and warm outside, and his writing is tortuous, round and powerful.