"modern" chinese was developed around the 1910s as a push for modernization/literacy as influenced by western imperialism/thought spreading into china. tldr they advocated for readability/accessibility, which meant a stylistic reform of the language.
if you can read modern chinese well enough, you can kinda understand about 30-80% of the meaning, depending how obtuse the text is and or when it was composed, but early chinese education (elementary school) includes some Tang poems from 600-900 AD, but is still quite readable. compared to something like middle english, which modern readers cannot read at all, you can see that the chinese language hasn't changed all that much
cf:
https://en.wikipedia.org/wiki/Classical_Chinese#Grammar_and_...