Short answers, pulled from the story.
Old East Slavic, also traditionally called Old Russian, was a language or group of dialects used by the East Slavs from the 7th or 8th century to the 13th or 14th century. It diverged into Russian and Ruthenian, and Ruthenian later evolved into Belarusian, Rusyn and Ukrainian.
Some scholars view the term Old Russian as anachronistic because it predates the dialectal divisions among the East Slavic languages. Alexander M. Schenker proposed the neutral term East Slavic, arguing that other names for the language had shifted along with political context.
Pleophony, or polnoglasie, meaning full vocalisation, is a sound feature that set Old East Slavic apart from other Slavic dialects. It turned Common Slavic's word for settlement into gorodъ, its word for milk into moloko, and its word for cow into korova.
The two most famous literary monuments are the Primary Chronicle, preserved in the Laurentian Codex of 1377, and the Tale of Igor's Campaign, known from a fifteenth-century Pskov manuscript. Other surviving texts include the legal code Russkaya Pravda and the prose poem Zadonshchina.
Regional differentiation accelerated after Kievan Rus' fragmented following 1100, and distinct regional languages became identifiable in the 12th or 13th century. After the end of the Tatar yoke, the territory split between the Grand Duchy of Lithuania and the Grand Duchy of Moscow, producing separate Ruthenian and medieval Russian literary traditions.
Alexander Vostokov and Izmail Sreznevsky undertook the earliest comprehensive lexicons of the language in the nineteenth century. Sreznevsky's Materials for the Dictionary of the Old Russian Language, published between 1893 and 1903, remained the standard reference until a 24-volume academic dictionary appeared between 1975 and 1999.