What happened to the Greek language was that the various dialects of Ancient Greek, including the highly prestigious Attic Greek, were superseded by koine. Koine Greek, the language of the New Testament, derived mostly from the Greek of Athens and simplified in a process described for other languages by John McWhorter in his Language Interrupted. All the Greek dialects of today derive from the Koine except for one. Everything that separates Ancient Greek from Modern Greek appears in the koine at the beginning of the first millenium and nothing like the drastic change from Old English to Modern English occurred after that. The written language was a mixture of old and new forms while trying to follow the Attic model. The spoken language bounced along, gradually developing some dialects but nothing extreme. There was no official language policy.
Once we enter the modern age, the forces of the Enlightenment and Nationalism enter the picture. Greek threw off Ottoman rule in the first half of the 19th century. Maintaining the classical language, or anyone’s version of it, was a powerful way of claiming a special status for the new nation. But as more and more people entered school, questions arose about which language was to be used, this highly artificial language based to some extent on the Ancient Greek model but highly mixed, or the popular speech used by everybody when they were not writing.
By the late 1800s, matters were becoming tense and by 1901 a translation of the Bible into the popular language provoked riots in Athens. By this time, the dialects that had arisen throughout the millenium were being swamped by the colloquial, spoken language and the time had come to codify this language if it were ever to replace the old written language. The terms “purist” and “populist” were coined (katharevousa and dimotiki from demos/people) and signaled the growing alienation of one pov from the other. The popular or dimotiki movement was called the “long hairs” and associated with Russian plots, which was exacerbated by the 1917 Russian revolution.
Subsequently the government vacillated back and forth, allowing more or less dimotiki in the elementary schools depending on the administration. Finally, a major blow was struck for the demoticists when the dictator Metaxas decided the dimotiki needed to be codified and commissioned such a work. In 1941 a demotic grammar was produced which then, ironically, became the platform from which to rigidify the code for using “the people’s language”.
What actually was going on was that the katharevousa or purist forms had seeped into the popular language to such an extent that few Greeks had any intention of speaking just dimotiki or just katharevousa; the popular idiom was a mixture. The dictatorship of 1964 to 1976 forced katharevousa on people for writing and formal speech and when it collapsed, diglossia essentially ended in Greece. But neither did people try to follow the dimotiki rules, which were based on old Greek folk songs. A more comfortable accomodattion seems to be in the offing with a gradual leveling and softening of the extremes.
This was a summary of Peter Mackrigde’s introduction to The Modern Greek Language. Anyone knowing more than this who can add to or correct this version, please do so in the comments.