There are three groups of adjectives in Black Speech:

1. short single-syllable adjectives, often attached to the words they describe;
2. polysyllable adjectives written separately;
3. formed from other words by adding suffix -ûrz ¹⁾ (thus also polysyllable standalone words);

Usually languages with extensive inflection of nouns have adjectives agreed with nouns in case, gender and number. But no signs of adjectives' inflection were found in Tolkien's Black Speech. The main reason is that short adjectives are usually attached to nouns as clitics (as in sharkû). In such situation adjective's ending affects noun's declension class and it's case and number suffixes are placed after a combined word. In Shadowlandian Black Speech where only unanimated nouns have number ending, adjectives describing animated nouns take suffix of number for clarification (but not case suffixes) anyway. Nûrlâm suggests that either both words must have no plural ending (classical or archaic variant) or both take it (modern / colloquial use). In proper syntax adjectives are placed after words they describe, but this rule is not strict.

In example, short adjective kû (old) attached to noun of 1st declension class, e.g. orn (tree), makes the resulting word belonging to 2nd declension class. If short adjective is written separately then it has no agreement in case with noun, the same as long adjectives.

Nûrlâm has two degrees of comparison: comparative (formed by suffix -ar) and superlative (made by adding suffix -az). Their suffixes are placed after derivative suffix -ûrz if it's present, but before plural suffix if used. If adjective is ended with vowel then suffix -z- is inserted before them. These forms are always written standalone even adjective's root consists of only one syllable. There is no special forms (like English “more”, “most”, “less”, etc.).

Comparative form is often used with conjunction snû (than) + <word in accusative case> or just <object in genitive>. E.g. the phrase “smaller than he” can be translated into Black Speech either as “gazar tab” or “gazar snû tash”.

• Wikipedia page about Adjectives.