We only know a few things about Adûnaic adjectives. First, they normally precede the nouns they modify (SD/428). Also, they do not agree in gender with nouns (SD/425), but there are several examples of plural adjectives that agree in number with the noun. Plural adjective seems to be formed using the same syntax as plural nouns.

In the situations where this is no agreement, (dulgu, katha), the adjective appears before the noun, modifying it directly.

• dulgu bawīb “*black winds”
• îdô katha batîna lôkhî “now all ways (are) bent”

In the situations where this is agreement, (dulgî, lôkhî), the adjective is the predicate of a copula (“to be” statement).

• bawîba dulgî “winds (were) black”
• îdô katha batîna lôkhî “now all ways (are) bent”

Perhaps plural agreement only occurs where the adjective is separated from the noun, as suggested by Carl Hostetter and Patrick Wynne (VSH/23). As they point out, the rules may have been different in the draft version of Adûnaic, since it has an example of a plural adjective directly modifying a noun: kathī batānī “all ways” (SD/312).

There are a couple of examples where adjectives are declined into the objective case, indicating that adjectives might sometimes be declined into other noun cases.

Finally, nouns that appear immediately before another noun can function adjectivally, in a genitive relationship with the following noun (SD/428), such as: kadar-lâi “city folk”. This means that there may not be much grammatical difference between Adûnaic nouns and adjectives. This could be the source of the plural-agreement rule discussed above.

Some authors have suggested that the suffix -ak in êphalak “far away” is an intensive suffix “very” for adjectives (LGtAG, NBA/32), similar in function to the Quenya prefix an-, but I think it is more likely a prepositional suffix “away”. See the entry on -ak for further discussion.