All living languages are constantly changing, as is clear from a comparison of documents in any given language written a few centuries apart. It is not always clear to native speakers that their language is changing around them, partly because the changes can be very slow and partly because changes in progress are perceived as dialect differences. However, Bill Labov (of the University of Pennsylvania) and his students have demonstrated that language change in progress can be observed and measured (Labov, 1994).
It's clear to everyone, including native speakers, that the vocabulary of a language changes over time. We can see that in English within this century new terms have been added (recently, "hip-hop", "Monicagate", "dweeb"), and also old terms have fallen into disuse ("groovy", "Tommy gun", "doughboy", "pinko"). But it's not just the vocabulary of a language that changes over time. All aspects of the grammatical structure of the language also change, as does pronunciation.
The linguists who constructed the Middle English corpus are interested in syntactic change, or the change in sentence structure over time. For example, here's a sentence from Malory's Morte d'Arthur illustrating a common Middle English structure that has largely vanished from Modern English.
Here is the sentence as Malory wrote it:
This is how the sentence appears, parsed and labelled, in the Middle English corpus.
( (IP-MAT (CONJ And) (NP-SBJ (NPR Merlion)) (BED was) (ADJP (ADVR so) (VAN disgysed) (CP-DEG (C that) (IP-SUB (NP-SBJ (NPR kynge) (NPR Arthure)) (VBD knewe) (NP-OB1 (PRO hym)) (NEG nat)))) (E_S ,)) (ID CMMALORY,30.939))
In this sentence, the negation "nat" appears at the end of its clause. In Modern English, the auxiliary verb "did" must be inserted and "not" must follow it immediately; we would say, "did not know him". This sentence could be written idiomatically in Modern English:
The documented stages of development of the English language are conventionally reckoned as follows:
|Old English||(ca.700 -- ca.1150)|
|Middle English||(ca.1150 -- ca.1500)|
|Early Modern English||(ca.1500 -- ca.1700)|
|Modern English||(ca.1650 -- the present)|
There are no sharp boundaries between the periods because language change is gradual and changes affect only small parts of the language structure at any given time, so that there's a great deal of continuity. The standard dates given here are used for the following reasons.
Our earliest surviving documents in English date from about 700; thus Old English is reckoned from that date. After the Norman Conquest (1066) writing in English declined rapidly, most official documents being written in French or Latin. Until about 1150, documents in English were still in the official Anglo-Saxon court dialect that had been developed before the Norman Conquest. In around 1150, the documents shift to colloquial dialects and the Anglo-Saxon court dialect disappears. 1500 is chosen as the end of the Middle English period because printing had been introduced into England in 1476, so that the conditions of survival of literary texts become very different from about 1500 on. From about 1700, documents in English are recognizable as fully modern in grammar.
It's easier to choose a boundary date when there are fewer documents, because you can see sharp distinctions between those few documents that survive. As time goes on and we have more documentation, the boundaries become blurrier. We have so many documents from the same period that we can see variation among them in the rate of change. This is why documents from around 1650 to around 1700 can be characterized as either Early Modern English or Modern English.
The documents from the early Old English period show a wide variety of colloquial dialects. Toward the end of the Old English period, the Anglo-Saxon court dialect (a literary language) was developed and became predominant. Then came the Norman invasion, and less and less English was written. Because so little English was being written, the court dialect tradition became weaker and weaker until it was finally abandoned. Then we begin to see documents written in colloquial dialects again for the next two centuries or more. This is the period called Middle English. It is interesting to linguists because it's the most recent form of written English in which these dialectal differences can easily be seen.
Toward the end of the Middle English period, the dialect of London began to acquire prestige. It became the new literary language. By the Early Modern English period, most writing in English south of Scotland was in the London dialect, and today the standard Modern English of every English-speaking country except Scotland is descended from that dialect. In our own time, the literary language of English has become completely standardized. You can tell if someone is from America or England or Austrialia or India when you hear them speak, but you probably could not tell from a writing sample (unless it happened to include one of the minor spelling and vocabulary variations.)