The first written texts of Middle English in the late 12th century show a remarkable simplification from Old English, just a century or two before. Verb and pronoun endings are reduced and simplified. Nouns lost their inflective endings (which indicate whether it was the subject or object of the verb) and relied more on prepositions and word order for meaning. Grammatical gender (nouns, adjectives and pronouns having different forms for male, female and neuter genders) was lost. And a few other simplifying changes that only a linguist could understand and appreciate. See this post by Linguistics Girl for a better explanation of these changes.
Linguists tell us that simplifications of this magnitude occur in a language only when it is learned as adults by a large enough group to influence the language spoken by the natives. It wasn't the Norman French, who were too few in number and mostly kept to themselves, who initiated this change. Rather it was the Norse from Denmark and Norway who had settled in eastern Britain in the late 8th and 9th centuries. This settlement (in contrast to previous Viking raids) was more or less peaceful. They ran businesses or farmed beside their English neighbors and often married English wives.
Because of their integration into English society, the Norse had to learn English. And adults, as you will know if you ever tried, find it difficult to learn another language fluently. Evidently they learned enough to get along and their simplified version of English gradually became the new standard across the country. Historical linguists tell us that these changes began in the northeast and gradually moved south over a period of several centuries.
A story related in Caxton's prologue to his translation of the Book of Eneydos in 1490 illustrates these changes in transition. A merchant ship was stranded in the Thames estuary by calm weather and a few of the men walked to a nearby house and asked the woman there for food. One asked for some "eggys", using the new (Norse) word. The woman didn't understand and said "Sorry I don't speak French", to which the merchant indignantly replied "Neither do I". Another merchant came to the rescue and explained that he was asking for "eyren", the old English word (singular "ey"), which the woman understood. You can read the story in Caxton's words here (page 2 starting line 25). He told the story to illustrate the predicament of English publishers in having to choose a dialect in which to print. It took over 200 years from the 14th to the 16th centuries for the word egg to become standard across England.
Norse also contributed many words to the English vocabulary. The most amazing are the pronouns they, them and their, replacing the Old English equivalents. Borrowing of pronouns is very rare and shows how significant the influence of Norse was on the core of the English language. Other core words include get, both, take, and want. Still other words are sky, skin, knife (and many other sk and kn words) and, as described above, egg. In many cases the Norse word was added, creating a synonym, rather than replacing the English word. Sometimes the adopted word took on a slightly different meaning, such as skirt (Norse) and shirt (English). Note that these words seem very English; they don't seem borrowed at all. Two reasons for this - Norse is another Germanic language so its words are more similar to English words than French or Latin words would be; and they were borrowed so long ago that they have become "Englishified" over time.
So English has two stages of people learning the language as adults and simplifying it in the process. First some people (possibly Semitic-speaking) in northern Europe learning Proto-Germanic (discussed in my previous post), then the Norse settlers in northeast England learning Old English. For them, all we native English speakers should be grateful, and those learning English as a second language even more so.