Old English (also called Anglo-Saxon) is an early form of the English language which was spoken in England around the year 1000. It is a West Germanic language, and is therefore similar to Frisian and Old Saxon. It is also quite similar to Old Norse (and, by extension, to modern Icelandic). Unlike modern English, Old English is a language rich with morphological diversity, and is pronounced essentially as it was spelt. It maintains several distinct cases: the nominative, dative, accusative, genitive, and instrumental, remnants of which survive only in a few pronouns in modern English speech.
Old English was not a static form. Its usage covered a period of some 700 or so years- from the Anglo-Saxon migrations into England in approximately AD 450, to some time after the Norman invasion in 1066, when the language underwent a major and dramatic transition. During this period of time, it assimilated some aspects of the indigenous pre-Celtic languages, some of the Celtic languages which it came into contact with, and some of the two variants of the invading Scandinavian languages occupying and controlling the Danelaw.
The most important shaping force on Old English was, of course, its Germanic heritage in vocabulary and sentence structure, shared with its sister languages in continental Europe. Some of these features were specific to the West Germanic language family to which Old English belongs, while some were presumably inherited from the Proto-Germanic language from which all Germanic languages are believed to have derived.
Though many of these links with the other Germanic languages have since been obscured by later linguistic influences, particularly Norman French, many remain even in modern English. Compare modern English 'Good day' with the Old English God dag, Dutch Goede dag, or German Guten Tag.
Like other West Germanic languages of the period, Old English was fully inflected with five grammatical cases, had dual plural forms, for referring to groups of two objects, in addition to the usual singular and plural forms. It also assigned gender to all nouns, even to inanimate objects: for example, seo sunne (the Sun) was female, while se mona (the Moon) was male.
The influence of Latin on Old English should not be ignored. A large percentage of the educated and literate population (monks, clerics, etc.) were competent in Latin, which was then Europe's prevalent lingua franca. It is sometimes possible to give approximate dates for the entry of individual Latin words into Old English based on which patterns of linguistic change they have undergone, though this is not always reliable. There were at least three notable periods of Latin influence. The first occurred before the ancestral Saxons left continental Europe for England. The second began when the Anglo-Saxons were converted to Christianity, and Latin-speaking priests became widespread. However, the largest single transfer of Latin-based words occurred following the Norman conquest of 1066, after which an enormous number of Norman words entered the language. Most of these oil language words were themselves derived ultimately from classical Latin, although a notable stock of Norse words were introduced, or re-introduced, in Norman form. The Norman Conquest approximately marks the end of Old English and the advent of Middle English.
The language was further altered by the transition away from the runic alphabet (also known as futhorc) to the Latin alphabet, which was also a significant factor in the developmental pressures brought to bear on the language. Words were spelt as they were pronounced; the silent letters of Modern English, therefore, did not often exist in Old English. For example, the hard-c sound in cniht, the Old English equivalent of 'knight', was pronounced. Another side-effect of spelling words phonetically was that spelling was extremely variable -- the spelling of a word would reflect differences in the phonetics of the writer's regional dialect, and also idiosyncratic spelling choices which varied from author to author. Thus, for example, the word "and" could be spelt either "and" or "ond".
Therefore, Old English spelling can be regarded as even more jumbled than modern English spelling, though it can at least claim to reflect some existing pronunciation, while modern English in many cases cannot. Most students of Old English in the present day learn the language using normalised versions, and are only introduced to variant spellings after they have mastered the basics of the language.
The second major source of loanwords to Old English were the Scandinavian words introduced during the Viking raids of the ninth and tenth centuries. These tend to be everyday words, and those which are concerned with particular administrative aspects of the Danelaw (that is, the area of land under Viking control, which included extensive holdings all along the eastern coast of England and Scotland). The Vikings spoke Old Norse, a language which is related to English in that they both derive from the same ancestral Germanic language. One theory holds that the presence of very similar words in both Old Norse and Old English helped accelerate the decline of case endings in Old English -- that is, if your Nordic neighbour says "horsu" and you say "horsa", you split the difference and just say "horse", reducing the ending to no more than a silent vowel. Others point out that the silent 'e' of English was pronounced up until the beginning of the Renaissance, so this compromise would be impossible. A compromise between "horsa" and "horsu" being "horse" is possible, but it would have a pronounced 'e'.
The number of Celtic loanwords is of a much lower order than either Latin or Scandinavian. As few as twelve loanwords have been identified as being entirely secure. Out of all the known and suspected Celtic loanwords, most are names of geographical features, and especially rivers.
To further complicate matters, Old English was rich in dialect forms. The four principal dialect forms of Old English were Northumbrian, Mercian, Kentish and West Saxon. Each of these was associated with an independent kingdom on the island. Of these, Northumbria and Kent were wholly overrun by Vikings during the 9th century. Most of Mercia was overrun as well, though a portion of it was successfully defended by and then integrated into Wessex.
After the process of unification of the diverse Anglo-Saxon kingdoms in 878 by Alfred the Great, there is a marked decline in the importance of regional dialects. This is not because they stopped existing: regional dialects continued even after that time, as evidenced both by the existence of Middle English dialects later on, and by common sense – people do not spontaneously develop new accents when there is a sudden change of political power.
However, the bulk of the surviving documents from the Anglo-Saxon period are written in the dialect of Wessex, Alfred's home kingdom. It seems likely that, with consolidation of power, it became necessary to standardise the language of government to reduce the difficulty of administering the remoter areas of the kingdom. As a result, paperwork was written in West Saxon. The Church was likewise affected, especially since Alfred initiated an ambitious programme to translate religious materials into the vernacular. In order to retain his patronage and ensure the widest circulation of the translated materials, the monks and priests engaged in the programme worked in his dialect. Alfred himself seems to have translated books out of Latin and into English, notably Pope Gregory I's treatise on administration, "Pastoral Care."
Due at least partially to the centralisation of power, and to the Viking invasions, there is little or no written evidence for the development of non-Wessex dialects after Alfred's unification.