There is the theory (mentioned in this paper [0], for example) that the critical event that pushed Old English on to the path of becoming modern English was not the Norman Conquest but the Danish invasions about two hundred years before.
As Germanic languages, Old English and Old Norse had a lot of similar vocabulary (e.g OE scirt, ON skirt, both meaning a unisex knee-length tunic, the former giving us the word 'shirt', the latter 'skirt'): but their grammar, especially their inflections, were different. Middle English, so the theory grows, developed from what was essentially a creole of Old English and Old Norse, which is why modern English does not have the complexities of inflection of, say, modern German.
As Germanic languages, Old English and Old Norse had a lot of similar vocabulary (e.g OE scirt, ON skirt, both meaning a unisex knee-length tunic, the former giving us the word 'shirt', the latter 'skirt'): but their grammar, especially their inflections, were different. Middle English, so the theory grows, developed from what was essentially a creole of Old English and Old Norse, which is why modern English does not have the complexities of inflection of, say, modern German.
[0] https://brage.bibsys.no/xmlui/bitstream/id/245485/Hanna%20Do...