What are the Old, Middle, and Modern periods of spoken Mongolian?

The history of the Mongolian language is categorized into three distinct periods of development:1

• Old (or Ancient) Mongolian: This period encompasses the language through the 12th century. Because no written records exist from this time, it is reconstructed by scholars through comparisons of recorded Mongolian languages and borrowings found in other languages.1
• Middle Mongolian: Spanning the 13th to the 16th centuries, this era corresponds to the Pre-Classical period of the written language. It is documented in various texts and glosses, including the Secret History of the Mongols (c. 1240), using Chinese, ’Phags-pa, Persian, and Latin scripts.1
• New (or Modern) Mongolian: This period began in the 17th century and continues to the present. Its commencement is associated with the conversion of the Mongols to Buddhism (c. 1575) and the subsequent Classical period of translation. Features of contemporary spoken Mongolian did not begin to appear in written texts until the 19th century.1