Few European languages have in the course of their histories undergone as radical changes as English did in the medieval period. The earliest documented variety of the language, Old English (c. 450 to 1100 ce), was a synthetic language, typologically similar to modern German, with its three genders, relatively free word order, rich case system, and verbal morphology. By the beginning of the Middle English period (c. 1100 to 1500), changes that had begun a few centuries earlier in the Old English period had resulted in a remarkable typological shift from a synthetic language to an analytic language with fixed word order, very few inflections, and a heavy reliance on function words. System-internal pressures had a role to play in these changes, but arguably they were primarily due to intensive contacts with other languages, including Celtic languages, (British) Latin, Scandinavian languages, and a little later, French. As a result, English came to diverge from its Germanic sister languages, losing or reducing such Proto-Germanic features as grammatical gender; most inflections on nouns, adjectives, pronouns, and verbs; verb-second syntax; and certain types of reflexive marking.
Among the external influences, long contacts with speakers of especially Brittonic Celtic languages (i.e., Welsh, Cornish, and Cumbrian) can be considered to have been of particular importance. Following the arrival of the Angles, Saxons, and Jutes from around 450 ce onward, there began an intensive and large-scale process of language shift on the part of the indigenous Celtic and British Latin speaking population in Britain. A general wisdom in contact linguistics is that in such circumstances—when the contact is intensive and the shifting population large enough—the acquired language (in this case English) undergoes moderate to heavy restructuring of its grammatical system, leading generally to simplification of its morphosyntax. In the history of English, this process was also greatly reinforced by the Viking invasions, which started in the late 8th century ce, and brought a large Scandinavian-speaking population to Britain. The resulting contacts between the Anglo-Saxons and the Vikings also contributed to the decrease of complexity of the Old English morphosyntax. In addition, the Scandinavian settlements of the Danelaw area left their permanent mark in place-names and dialect vocabulary in especially the eastern and northern parts of the country.
In contrast to syntactic influences, which are typical of conditions of language shift, contacts that are less intensive and involve extensive bilingualism generally lead to lexical borrowing. This was the situation following the Norman Conquest of Britain in 1066 ce. It led to an influx of French loanwords into English, most of which have persisted in use up to the present day. It has been estimated that almost one third of the present-day English vocabulary is of French origin. By comparison, there is far less evidence of French influence on “core” English syntax. The earliest loanwords were superimposed by the French-speaking new nobility and pertained to administration, law, military terminology, and religion. Cultural prestige was the prime motivation for the later medieval borrowings.
Some Breton Words in the Dictionary of the Russian Empress