[pierrem15]

Actually, the one change that contributed most to the collapse of the declension system in English was not the Norman invasion but the Viking one.

Old English had a large amount of mutual intelligibility with Old Norse, but the declensions in certain cases were different. So people stopped using them and replaced them with word order and prepositions. The same thing has occurred in Norway where certain dialects with slightly different declensions came to overlap and a common descendant formed with reduced declensional structure.

[Paladin2]

My Grandma exhibited common sense.

[SunkenCiv]

Suffice to say, the path from old to current English had many merged forks.

[Verginius Rufus]

Some basic words in English are Scandinavian, like they/their/them and skirt (shirt is from Anglo-Saxon).

The closest relative of English is Frisian, spoken in the Netherlands.