Old English:

Another source of loanwords was Old Norse, which came into contact with Old English via the Scandinavian rulers and settlers in the Danelaw from the late 9th century, and during the rule of Cnut and other Danish kings in the early 11th century. Many place-names in eastern and northern England are of Scandinavian origin. Norse borrowings are relatively rare in Old English literature, being mostly terms relating to government and administration. The literary standard, however, was based on the West Saxon dialect, away from the main area of Scandinavian influence; the impact of Norse may have been greater in the eastern and northern dialects. Certainly in Middle English texts, which are more often based on eastern dialects, a strong Norse influence becomes apparent. Modern English contains a great many, often everyday, words that were borrowed from Old Norse, and the grammatical simplification that occurred after the Old English period is also often attributed to Norse influence.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". What led to more complicated grammar after the Old English period?
unanswerable