Instrumental Case was used to express instrumental meaning but only in the adjective while the noun stood in Dative Case:
by/with + Adjective (Instr) + Noun (Dat)
Degrees of Comparison –positive, comparative, superlative.
Determination (Definiteness/Indefiniteness) –today this category has to do with the Article but in OE there were no articles and definiteness/indefiniteness was expressed with the help of inflections of the Adjective, i.e. the inflections of the Adjective helped to determine whether a noun was definite or indefinite.
Historically the Adjective is a younger class of words as compared to the Noun. So it has borrowed many of its categories and inflections from the Noun and the Pronoun.
The Adjective had the following categories:
Gender
It still existed in OE but was the first category to disappear in the 11th c.
Case
· At the end of OE Period Instrumental Case fell together with Dative Case due to the homonymy of inflections;
· All other cases disappeared by the end of the 13th c. also due to the homonymy of inflections.