Short answers, pulled from the story.
Germanic tribes known as Angles, Saxons, and Jutes crossed the North Sea to settle in Britain during the mid-5th century. Their arrival marked a turning point where Anglo-Saxon dialects replaced Common Brittonic and Latin across much of England.
Old English featured four main dialects including Mercian and Northumbrian which formed the Anglian group north of the Thames and Humber River respectively. West Saxon dominated south and southwest of the Thames while Kentish occupied the smallest southeastern corner settled by Jutes from Jutland.
Nouns in Old English declined for five cases: nominative, accusative, genitive, dative, and instrumental. They also carried three genders, masculine, feminine, neuter, and two numbers: singular and plural.
Scribes added four new characters to the Latin alphabet: eth (ð), thorn (þ), wynn (wynn), and ash (æ). Eth first appeared in dated materials during the 7th century while thorn emerged in the 8th century.
Cædmon's Hymn stands as the oldest surviving work composed between 658 and 680 but written down only in the early 8th century. Cædmon served as a lay brother in the monastery at Whitby before composing his famous hymn.
Scandinavian rulers and settlers began influencing Old English from the late 9th century onward during Danelaw rule. The eagerness of Vikings to communicate with Anglo-Saxon neighbors created friction that eroded complicated inflectional endings leading to simplified grammar.