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 (mostly nouns) 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 (standard variant) or both take it (modern / colloquial use).

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.

Keep in mind, that suffix -ûrz is supposed to form adjectives from nouns. Verbal adjectives (like English “stinky”) should be translated as participles (“pushdug”, see Orcish curse) or gerundive (“pushdat” = lit. “stinkable”).

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 “mikar tab” or “mikar snû tash”.

While Shadowlandian dialect suggests that all adjectives should be placed after nouns in proper syntax, in Nûrlâm it's considered archaic. All Tolkien's examples of this rule (Lugbûrz, sharkû) are combined words where short adjectives become clitic. So Nûrlâm assumes that long stand-alone adjectives (especially comparatives) may be placed before nouns. When long adjective is placed after noun in Black Speech text, it usually means that verb “kulat” (to be) is omitted between them. However many examples throughout this wiki have adjectives placed after nouns due to influence of Shadowlandian and stylistic reasons (e.g. avoiding further similarity to English). Short adjectives are often attached after noun's root. Short adjectives are written separately if there are more than one describing the same word, especially when joined by word “agh” (and).

• Adjectival phrases
• Wikipedia page about Adjectives.