At some point between the late Proto-Germanic and Proto-Northwest Germanic, the Instrumental and dative plural endings had begun syncretizing.[1]
The second-person plural *jūz and dual *jut pronouns were adjusted to *jiz (compare Old Norse èr, Old english ġē, Old saxon gē~gī) and *jit (compare Old Norse it, Old English ġit, Old Saxon git) under the influence of the first-person *wiz and *wit.[1]
In Proto-Germanic, present participles were consonant stems which ended in -nd-, with feminines ending in -ndī~-ndijō. In Proto-West-Germanic, masculine and neuter present participles regardless of gender ended in -ndija-, which was probably backformed from the feminines.[2]
A small number of inherited neuter collectives like *sēmô "seed" (cognate with Latin sēmencode: lat promoted to code: la ), *namô "name" (cognate with Latin nōmencode: lat promoted to code: la , Ancient Greek ὄνομα) were transferred to masculine singulars in the n-stem class.[3]
The third-person pronoun *i-~*e- was replaced with *hi-~*he- in the southern West Germanic dialects.[4] Additionally, the feminine nominative singular *sī acquired the usual feminine ending *-u, becoming *siju (however, *sī is attested in Old High German).[5]
The demonstratives *sa (masculine nominative singular) and *sū (feminine nominative singular) were replaced with *siz and *siju under the influence of the third-person pronoun.[5]
Strong verbs whose roots ended in *f had their Verner alternations destroyed when *[β] and *f merged into v word-internally, and into f word-finally (both [f] and [v] were written f).[6]
The instrumental case merged into the dative case.[7]
Nominative and accusative plurals merged in nouns.[8]
Ringe, Donald; Taylor, Ann (2014). The Development of Old English – A Linguistic History of English, vol. II. United States of America: Oxford University Press.