Short answers, pulled from the story.
The Sukhothai Kingdom was formally established in 1238 CE, when a local chieftain named Bang Klang Hao, later known as Si Inthrathit, recaptured the city from a Monic faction and re-established it as an autonomous Siamese polity. He founded the Phra Ruang dynasty and ruled until 1270.
Sukhothai derives from the Sanskrit-Pali compound of sukha, meaning happiness, and udaya, meaning dawn, producing translations such as "the dawn of happiness" or "rising happiness." The name is also explained as "The Happy Thai."
Ram Khamhaeng the Great ruled Sukhothai from 1279 to 1298 and expanded the kingdom to its greatest territorial extent, with claimed suzerainty stretching from Luang Prabang in the north to Nakhon Si Thammarat in the south. He is credited with introducing Theravada Buddhism as the state religion, likely inventing the Sukhothai script in 1283, and opening diplomatic and trade relations with Yuan China that produced the export of sangkhalok ceramics.
The Sukhothai Kingdom ended in 1438, when Borommapan, its last ruler, died and Borommarachathirat II of Ayutthaya installed his son Ramesuan as Upparat, a position combining viceroy and heir presumptive, effectively annexing the kingdom. Sukhothai had been a tributary state of Ayutthaya since an invasion in 1349.
The Ram Khamhaeng Inscription, also known as Inscription No. 1, is the earliest known evidence of the Sukhothai script, which Ram Khamhaeng likely created in 1283. It was discovered by Mongkut, Rama IV, nearly six centuries after it was carved, and its account of heroic kings and the kingdom's administrative life became the foundation for incorporating Sukhothai into Thai national history in the late 19th century.
The former Sukhothai territories, called the Northern Cities by Ayutthaya, continued under local aristocrats within the mandala system and gradually merged culturally and politically with Ayutthaya during the 15th and 16th centuries. Sukhothai nobles served as kingmakers in Ayutthayan succession conflicts, and Sukhothai military leaders were prominent in Ayutthaya's army. After the Battle of the Sittaung River in the Burmese-Siamese War of 1584-1593, Naresuan the Great forcibly relocated people from Phitsanulok, Sukhothai, Phichai, Sawankhalok, Kamphaeng Phet, Phichit, and Phra Bang closer to Ayutthaya.