Modern English is a mixture of many languages. One of the early contributors was the Vikings or Norsemen, who spoke Old Norse. Their raids of England began in 793 with the robbing of the monastery Lindisfarne and lasted more than 200 years. During this time much of England was either under Viking rule or living in fear of them. People prayed to God, "From the fury of the Norsemen deliver us, O Lord."
Alfred the Great, originally crowned King of Wessex in 871, became the first king of all the English*. He fought the Danish Vikings and freed southern England. However by 1016, the Danish Viking Canute invaded England and made himself king. In 1066 the Vikings were finally defeated by King Harold II at the Battle of Stamford Bridge. (A few weeks later the Normans, who were French-speaking Viking descendants, invaded and ruled until 1485, but the blending with French is another story.) As a result of these invasions Old English was heavily mixed with Old Norse.
Many new words were added to Old English. Old Norse and Old English were related languages. Similar sounding words with slightly different meanings were created from mixing Old English with Old Norse: shirt/skirt, skin/hide, skill/craft, cut/carve.... Most English words beginning with sk- in Modern English are from Old Norse. There are many times that it is difficult to figure out where our modern word comes from since the Old Norse and Old English were so similar. Is the verb 'meet' from the Old English 'metan' or the Old Norse ? Old Norse competed with Old English for which word would be used. The Old Norse word 'leggr' became the Modern English 'leg' and replaced the Old English word 'shank'. Likewise 'egg' replaced . The mixture made English vocabulary richer.
Many common everyday English words come from Old Norse. They cover almost the whole alphabet: anger, bag, crawl, die, egg, freckle, get, husband, ill, kick, leg, mistake, nag, odd, plough (US plow), queasy, rid, seem, shout, take, they, ugly, Viking, wrong and Yule. Only words beginning with j, x or z are free from Old Norse, because English did not use those letters back then. I could fill this page with English words that came from the Vikings. Consider this simple sentence where many of the words come from Old Norse.
On Thursday I cut (kuti) a cake (kake) with my knife and gazed (gapa) out of the window (vindauga = 'wind eye') at the sky (skie).
Like all related languages from England to India, Old English had many inflections, changes to word endings to show different grammatical uses. Plural forms, adding -s or -es to the end of words, are inflections. Plurals are one of the few English inflections to survive. The confusion of blending the Old English and Old Norse languages forced people to simplify the language and drop most of the other endings. Only a few Modern English words still keep the complicated Old English inflections, such as mouse/mice, foot/feet, ox/oxen....
Modern English is a blend of languages. This blending has been going on ever since the English first invaded Britain a thousand five hundred years ago. Britain was invaded many times and very often lived under colonial rule. After a long time the English started travelling and met new languages around the world. The blending with Old Norse forced great simplifications of Old English grammar. Many of the easiest and simplest Modern English words come from Old Norse.
*England was slowly united over several generations, so people do not always agree who was the first King of England. Alfred the Great was the first king to have the support of all the English people as a nation.