Old Tibetan
Old Tibetan refers to the period of Tibetan reflected in documents from the adoption of writing by the Tibetan Empire in the mid-7th century to works of the early 11th century.
In 816 CE, during the reign of Sadnalegs, literary Tibetan underwent a thorough reform aimed at standardizing the language and vocabulary of the translations being made from Indian texts, and this resulted in what we now call Classical Tibetan.
Phonology
Old Tibetan is characterised by many features that are lost in Classical Tibetan, including my- rather than m- before the vowels -i- and -e-, the cluster sts- which simplifies to s- in Classical Tibetan, and a reverse form of the "i" vowel letter (gi-gu).
Case system
Case morphology is affixed to entire noun phrases, not to individual words (i.e. Gruppenflexion). Old Tibetan distinguishes the same ten cases as Classical Tibetan:
absolutive (morphologically unmarked)
genitive (གི་ -gi, གྱི་ -gyi, ཀྱི་ -kyi, འི་ - 'i, ཡི་ -yi)
agentive (གིས་ -gis, གྱིས་ -gyis, ཀྱིས་ -kyis, ས་ -s, ཡིས་ -yis)