Proto-Germanic adjectives are declined according to number (singular and plural), case (nominative, vocative, accusative, genitive, dative and instrumental) and gender (masculine, feminine and neuter). The number, case and gender of each adjective matches that of the noun or pronoun it modifies.
As with nouns, adjectives can be divided into several declension classes based on the formation of the endings. However, there are less distinct classes, and the classes themselves are mostly alike in their declension. A distinct characteristic of Germanic adjectives however is that each adjective can be inflected according to two different paradigms: strong and weak. The strong inflection is that of the adjective itself, and is either a-stem, i-stem or u-stem. The weak inflection is always the n-stem declension.
The two kinds of inflection reflect a difference in meaning. Weak adjectives are considered definite, and refer to specific individuals, known people or things, or to people or things for which the adjective is a defining characteristic. They are often used with demonstrative determiners, although they can also be used alone, in which case they are similar to a definite article. Strong adjectives are used in all other cases. Many determiners can inflect only according to one type, and are either always strong or always weak.
The a-stems are by far the most common type of adjective in Proto-Germanic. They inflect similar, but not quite the same as the a- and ō-stems of nouns. There are also ja-stems and wa-stems, which are more rare.
The i-stem adjectives are more rare. They are almost identical to ja-stem adjectives, and differ only in the masculine and feminine nominative singular, and in the neuter nominative and accusative singular.
The u-stems are also rare, and are analogous to i-stems. Like i-stem adjectives, they are declined almost identically to ja-stems, except for the nominative singular.
The n-stems represent the weak adjective inflection, and are not a distinct class of adjective. However, some determiners always inflect as weak, and are never strong. The inflection is identical to the an- and ōn-stem declension of nouns.
Comparatives and ordinals use an alternative variety of the weak inflection. In this inflection, the feminine forms are not those of the ōn-stem nouns but of the īn-stems.
Germanic adjectives had both a comparative and superlative form. The comparative was formed with -izô or -ōzô and always followed the an/īn-stem declension. The superlative was formed with -istaz or -ōstaz and declined like a regular a-stem adjective, either strong or weak. Which variety of the comparative or superlative suffixes was used, with -i- or -ō-, depended on the declension of the original adjective but was also random and unpredictable. The ja-stem, i-stem and u-stem adjectives always used -i-. Regular a-stem adjectives used either, and which suffix was used for a particular adjective could not be predicted easily, it had to be memorised for each adjective.
There was also a rarer comparative form in -umô, which declined like a regular comparative. It was mostly found in old relic formations and was probably only marginally productive. It had a corresponding superlative in -umistaz.
Comparatives were originally separate adjectives, and this still shows in the behaviour of some adjectives, which form their comparative and superlative with a different stem from the positive.
The most extreme examples are what is called "suppletive comparison", where the entire root was different. Sometimes, the comparatives remained in a daughter language while the basic adjective was replaced, showing that the paradigmatic connection between the degrees was not strong, and the positive degree could easily be substituted. In this way, several adjectives may have shared the same comparative, as in English many and much today. The word "small" even seems to have had two comparatives originally, but it's not clear whether there was a difference in meaning. There is also a single Gothic example of a possible alternative comparative for "good", but it is not found anywhere else in Germanic.
|*gōdaz (“good”)||*batizô (“better”)||*batistaz (“best”)|
|*iusizô (“better”) ?||*iusistaz (“best”) ?|
|*ubilaz (“bad”)||*wirsizô (“worse”)||*wirsistaz (“worst”)|
|*mikilaz (“great, much”)||*maizô (“greater, more”)||*maistaz (“greatest, most”)|
|*lītilaz (“small, little”)||*minnizô (“smaller, less”)||*minnistaz (“smallest, least”)|
|*laisizô (“smaller, less”)||*laisistaz (“smallest, least”)|
In a few cases, Verner alternation occurs between the positive and the other two degrees, with the positive having the voiced alternant and the comparative and superlative the voiceless alternant. In the case of "old", the positive degree was lost in Old Norse and replaced with gamall, creating a new suppletive paradigm.
Finally, there are also some cases in which a suffix that was present in the positive degree was dropped in the comparative and superlative degree. This is probably another example of how the comparative system was originally derivational rather than paradigmatic (in which case one suffix simply replaced another). One example is known from Gothic: