That’s what I learnt as well. The Vikings took over approximately half of England for two centuries, in the area that became known as the Danelaw. Old Norse and Old English were cognate languages, but the endings of nouns were different, so in a process of amalgamation, the -s plural became standard. One reason for this was the ~ two centuries of the Danes in north-east England, prior to the Norman Conquest, and also the fact that there were a lot more Danes that invaded and settled than Normans.
In 886, the Treaty of Alfred and Guthrum was formalised, defining the boundaries of their kingdoms, with provisions for peaceful relations between the English and the Vikings. The language spoken in England was affected by this clash of cultures, with the emergence of Anglo-Norse dialects.[5]
