Complex sentence: In China, the work had been well-known as a text for teaching Chinese to read and write Mongolian during the Ming Dynasty and the Chinese translation was used in several historical works, but by the 19th century, copies had become very rare.
Split into simpler sentences: In China, the work had been well-known as a text for teaching Chinese to read and write Mongolian during the Ming Dynasty and the Chinese translation was used in several historical works. But, by the 19th century, copies had become very rare.