System of Declensions. The system of declension was inherited from PG

The system of declension was inherited from PG. Adjectives had two declensions that had to do also with the category of determination – strong (definite) and weak (indefinite) – and unlike nouns practically all adjectives could be declined both ways (by strong and weak declension). So an adjective did not belong to a particular declension, its declension depended on several factors that will be mentioned below:

There were exceptions from the rule: some adjectives were declined always strong (eall (all), maniζ (many), ōþer (other)), others – always weak (ilca (same)).

The endings of the adjectives showed the agreement between a noun and an adjective. There were a lot of homonymous forms (e.g. -um (OE) – N, Sg, Dat, strong; N, Pl, Dat, strong; N, Pl, Dat, weak; N, Pl, Instr, strong; N, Pl, Instr, weak; -e (ME) – N, Pl, strong; N, Sg, weak; N, Pl, weak) à the distinction between the declensions faded in ME and the declensions disappeared as far as there was no necessity any more to keep them.

Number

There were some homonymous forms in Singular and Plural in both declensions (see the table above: e.g. -um (OE) – N, Sg, Dat, strong; N, Pl, Dat, strong; -e (ME) – N, Sg, weak; N, Pl, weak), so the category of Number disappeared together with the system of declensions.

The Adjective lost many of its categories in ME as far as all the inflections were lost. Thus it became an unchangeable part of speech.