How did *Old* English transform into *Middle* English so quickly?

Anglo Saxon Old English was the most common language in England before the Norman invasion. To the modern eye, it is unintelligible without specialist learning:

lange þrage;     he him ðæs lean forgeald.
II
Gewat ða neosian,     syþðan niht becom,
hean huses,     hu hit Hring-Dene
æfter beorþege     gebun hæfdon.

Geoffery Chaucer wrote The Canterbury Tales about four hundred years later. It’s awkward for the modern reader, but essentially understandable:

Whan that aprill with his shoures soote
The droghte of march hath perced to the roote,
And bathed every veyne in swich licour
Of which vertu engendred is the flour;

Languages evolve of course, but usually not so fast. Six hundred years later, we’re still speaking roughly the same language as Chaucer with the same grammatical rules.

Back then, how did the language transform itself so completely from Anglo Saxon Old English in such a short time? I’d like some details, if possible, not just “because of the Norman invasions”.

There was no single Anglo-Saxon Language before the Norman Invasion. Baugh & Cable, (A History of the English Language , 4th edn, 1993) tell us that there were four dialects. The two north of the Thames, Northumbrian and Mercian had enough similarities to be collectively referred to by some as Anglian. The two southern dialects were West Saxon and Kentish.

By the time of the Norman invasion, these dialects were very different from what they had been four centuries before. This is hardly surprising. Very few people indeed could read or write, and it is the written word that tends to slow down change in language, a change in the dialects that would become English that had been accelerated by the influence of Viking invasions and of church Latin.

The English dialects after the Norman invasion were largely spoken by the peasants and serfs, Norman-French and Latin being the languages of the ruling class and the majority of literate people.

By the time English began to become the language of all classes in the middle ages, the influence of Norman-French had made a considerable difference to both the grammar and vocabulary of the previous largely Germanic language.

Whilst the language of Chaucer was clearly more like modern English than was, for example, the West Saxon of four centuries before, it is not really true that "essentially understandable" for the modern reader. I doubt if many people today could understand the first four lines of the Canterbury Tales that you quoted. Some of us can, because we studied it at school or university, but that does not mean that everybody can. This would be even truer of the spoken language, which would sound like a foreign language to most native speakers of modern English.

Even the language of Shakespear, two hundred years after Chaucer and a mere four centuries ago is not easy, as these lines from Macbeth show: If it were done, when 'tis done, then 'twer well,it were done quickly: If th' Assassination could trammell vp the Consequence, and catch with his surcease, Successe: that but this blow might be the be all, and the end all. Heere, but heere, vpon this Banke and Schoole of time, wee'ld iumpe the life to come. But in these Cases, we still haue iudgement heere, that we but teach bloody Instructions, which being taught, returne to plague th' Inuenter.

A person who had not studied Shakespeare at school, or seen a play would find that fairly difficult.

The spread of the printed word after Caxton's introduction of the printing press towards the end of the fifteenth century did help fix, to an extent, the spelling and grammar of the language. The approach to uniformity in the language was also helped by the stability, compared to earlier centuries, of central government, the absence of foreign invasion and, by the end of the nineteenth century, the introduction of universal education in Britain.

So, it may be that, in the four centuries between 1000 and 1400 the language changed more than it did between 1600 and 2000, but the reasons are reasonably clear. I think it is also clear that the earlier change was not so drastic, nor the second so slight, as you suggested in your question.

Old English and Anglo-Saxon are the same thing ("Traditional histories of the English Language have divided their account into three major periods: Old English (sometimes refered to as Anglo-Saxon), Middle English, and Modern English" ~ A History of the English Language N.F. Blake p5) and arguments that it didn't exist are not linguistic, nor are they (as some seem to believe) arguments against a single language existing. A single language existed, it was used in the 7 versions of the Angl-Saxon Chronicles, and under Alfred became the first recognised Standard English as England as a political identity was formed.

The 'no such thing as Anglo-Saxon language' argument is basically one of labelling. The divisions of Old, Middle, and Modern English came from Henry Sweet in 1873, and he based them on inflectional characteristics. OE had full inflections; ME had levelled inflections; and Modern Eglish lost them. This is flawed. OE had levelled inflections and Modern has retained inflections, so there is no natural, datable, division. This is the argument against the concept of Old English/Anglo-Saxon as a language: when did people start speaking it, and when did they stop? All we can really say (which is Tolkiens argument) is that there was a language spoken in the arbitrary period that we label as the Old English or the Anglo-Saxon period, but the language should be called just Englisc as it has no connection with the historic labels. Basically, Anglo-Saxon language didn't begin or end with the Anglo-Saxon period, and of course they didn't describe themselves as Anglo-Saxon speakers, so it should have a different name.

Turning to the 'abrupt' change to Middle English, it was not abrupt at all. It simply appears that way because of the general lack of texts recording the language, and because the texts all come from a small elite of educated, literate individuals and was (just as today) quite different to language used by the general population. Also, like any language change, it took time to spread so OE and ME were in use together for an extended period.

The apparent abrupt transition is due to a great discontinuity in the written language as a direct result of the Norman conquest; spoken English would have been on a much smoother course of change (albeit still accelerated by social and political disruptions).

Post-Alfredian Old English had a literary standard based on the West Saxon dialect. Written standard languages are, by design, conservative; their use masks most of the dialectal features present in the spoken language.

The Wessex standard abruptly lost its status after the conquest; from about 1100 to 1400, there was no standard written English; texts from that time basically show the language as the authors spoke it.

Old English, roughly from 450 to 1150, kept its form for 700 years. I don't think that it changed so quickly into Middle English.