Most authors like Baugh (1993) have set the start of the Middle Age period in 1150, but many others like wheeler (2006) agree that the starting point must have been marked by the breaking point of the Battle of Hastings, that is, 1066. Although none of the two dates is accurate enough, that would be a good start to understand the circumstances that lead us to assure a change in the language.

Analogic ways of phonetic changing resulted on reduction of inflections. Number, case, and gender inflections were lost because of an alteration in their pronunciation. This is also true of the verb. A number of endings like -a, -u, -e, -an,and -um were replaced by what is called the 'indeterminate vowel', that is, they were all reduced to te ending -e.

Middle English to the detail:

The Noun
In early Middle English there were two ways of making the plural: -s or -es for the strog declension and -en for the weak. Up until the 13th century, the -en ending was the most used in the south, but not in the north. By the year 1200, the -s ending had taken over. The reason for this change may have been the similarities between the -s ending , and the plural ending of Anglo-Norman, which was also -s.

The Adjective
The nominative singular form was extended to all cases of the singular, and the form of the nominative plural to all the cases of the plural. That also made the ending -e for both the singular and the plural, so there was not a difference between them anymore.

The Pronoun
The loss of inflections meant that one could not depend on the formal indications of gender case and number, so it made it necessary to rely on word order. this made a drastical change on syntax (word order).

Another simplification to take into account is that of the loss of the dual number.

The Verb
The main changes in theverb during this period were the losses suffered by the strong conjugation. Many of the verbs that survived the assimilation of Anglo-Norman changed over to the weak inflection. Nearly a third of the strong verbs in OE were lost in the ME period. They either dissappeared or became weak.