In Indo-European languages generally, the inflection of the adjective changes to reflect the grammatical characteristics (gender, case and number) of the noun it is modifying. Thus the ending we find on the adjective will be different depending on whether it is modifying a masculine nominative singular noun, a feminine nominative singular, a neuter accusative plural, a neuter genitive plural, and so on. As a result, even though the adjective system of Old English is much simpler than that of many other Indo-European languages, the number of endings that can be applied to an Old English adjective is larger than the number that can be applied to an Old English noun.
Further, most adjectives in Old English (and the other Germanic languages as well) can belong to two rather different declensions, depending on the context in which we find them. An adjective will have the endings of the "strong" declension (so called because the number of "strong" endings is relatively large) when no pronoun or possessive adjective precedes it. It will have the endings of the "weak" declension (so called because the number of "weak" endings is relativelly small) when it is preceded by a pronoun or possessive adjective. For example, here are two paradigms for the adjective god "good" with the feminine noun cwen "queen." First the strong:
|nominative||god cwen||gode cwena|
|accusative||gode cwene||gode cwena|
|genitive||godre cwene||godra cwena|
|dative||godre cwene||godum cwenum|
And now the weak:
|nominative||seo gode cwen||þa godan cwena|
|accusative||þa godan cwene||þa godan cwena|
|genitive||þære godan cwene||þara godra cwena|
|dative||þære godan cwene||þam godum cwenum|
The adjective oþer "other, second" is always strong (for some reason), and a comparative adjective is always weak. In verse, weak adjectives may occur without being preceded by pronouns or possessive adjectives, but it is virtually unknown in either verse or prose for a strong noun to be preceded by a pronoun or possessive.
As in Modern English, the comparative adjective is normally made with -r- and the superlative with -st-.
Here are a few facts that will ease your way. First, notice the resemblance between the declension of the weak adjectives and that of the weak nouns. As a rule they look the same, except that the genitive plurals are different, Second, notice the resemblance between the endings of the strong adjectives and those of most pronouns. You'll find it a big help to remember (for example) that feminine genitive and dative singular strong adjectives and pronouns all end in -re. If you study the paradigms in your textbook you will turn up other resemblances as well. Third, remember that the declension of the noun does not vary according to context. A noun is either strong or weak no matter where it comes.