Old English adjectives declined based on case, gender, and number to agree with the noun they described. There were two main declension patterns - weak and strong. In the singular, weak adjectives declined the same for all genders, while strong adjectives had different endings for masculine, neuter, and feminine nouns. In the plural, all adjectives took the same endings regardless of their declension. Comparatives were formed with suffixes like "-ra" and superlatives with "-ost", though some irregular forms also existed.