The replacement of Old English pronouns "hie," "hem," and "heora" with "they," "them," and "their" was influenced by Old Norse.
During the Viking invasions and the subsequent settlement of Norse-speaking people in England (especially in the Danelaw region), Old Norse and Old English coexisted and influenced each other. The Old Norse pronouns "þeir" (they), "þeim" (them), and "þeira" (their) gradually replaced the native Old English forms due to their similarity and frequent interaction between the two languages.
This is one of the most significant grammatical influences of Old Norse on English.
