What are the factors that have led to the change from Old to Modern English?
There are both external and internal factors that help explain the change from Old English to Modern English. External factors include who spoke the language and any "political and social factors" that drove change. Internal factors include motivations to structure the language in terms of "punctuation, grammar, vocabulary and written appearance" (Lohr, "From Old English to Modern English").
The period of Old English, also called Anglo-Saxon, dates back to about 450 AD when West German tribes, called the Angles, Saxons, and Jutes, settled in southern Britain. The Germanic language served as the basis for English vocabulary and irregular verbs. Prior to the invasion of the Anglo-Saxons, Rome invaded Britain, starting in 55 BC, leaving significant Latin influence, but that Latin influence was augmented when the Romans introduced Christianity to Britain in the 7th century ("The Romans in Britain"; Lohr). Old English became mostly written in the Latin alphabet with a few Germanic letters included. Viking invasions further influenced Old English vocabulary.
The period of Modern English begins with the invention of the printing press in the 1400s. The development of printing brought with it a desire to also develop a "standardized variety" of English. William Caxton set up the first prominent printing company in England in 1472, and the books he published helped set a precedent for using the "East Midlands/London variety of English" as the standard form of English (BBC, "William Caxton (c. 1422 - 1492)"; Lohr). Soon, the Johnson's dictionary and grammar books published in the 1700s further helped to codify standard English.
Not only did the Vikings further the vocabulary of English with such words as husbondi (husband) and syster (sister), but the Danes added pronouns, filling a real linguistic need. Also, since Old English had noun declensions like German, the Danes simplified language by eliminating this practice, but it was Middle English which completed this process by using noun position in a sentence.
Old English was in usage from 449-1066. In 1066, William the Conqueror of Normandy defeated King Harold in the Battle of Hastings and French became the official language of the island of England because the invaders considered themselves far superior to the Saxons and Danes. Official documents were written in French; the tales of King Arthur were written in French. To this day a very high percentage of English words have French derivatives, especially in law, politics, religion and the arts. It was Geoffrey Chaucer, having written The Canterbury Tales in Middle English, who re-introduced English into literature and other important writings. Middle English was in use until the fifteenth century. Middle English reduced some grammatical cases, simplified noun and adjective inflections, and also simplified verb conjugations. The language used from 1470 to 1650 is considered Early Modern English. Linguistic authorities contend that Shakespeare composed his works in this Early Modern English, which has a largely Anglo-Saxon vocabulary. One of the greatest changes to Early Modern English was the loss of inflectional endings the change in verb conjugation.