Besides the official rules of vowel/consonantal combinations, here is a list of other rules for forming words.
(Fair warning, these are deductions made by the community based off patterns in his works, so we will never know for certain whether or not this was what the renowned author would have wished).
Onomatopoeia.
This means that when it comes to animals, sounds, or various objects with sounds associated with them, their name should evoke memories of that sound (such as karab for horse, which evokes the thought of a horse galloping).
Exceptions to the rule is where the creature or object is one naturally shunned and rather not talked about, then it tends to have a name identical or nearly so in sound to an elvish word for it (such as uruk for “orc”).
A corollary to this is the association of -u- and -ô- with stronger/darker themes, while -i- and -ê- tend to be with lighter, more feminine themes.
No more than three consonants!
Adûnaic is a composite bi/triconsonantal word system. In other words, the base word cannot have more than three consonant sounds. The only way to bypass the limit is to either form a word from multiple words or add a suffix onto the base word (such as agental and verbal suffixes).
Example: pharaz + -ôn (“gold” + adjectival suffix) à pharazôn (“golden”). huzun + -dâ (“ear” + verbal suffix) à huznudâ (“to hear”). Katha + phazag- + -ân (“all” + “to rule” + agental suffix) à kathuphazgân (“conqueror, (lit.) all-ruler”).
Standalone Suffixes.
Words meant to be used as a suffix should by default be only one short vowel, (unless you have an -e- or an -o-, which in that case it’s long by default).
If used as a standalone word, the base vowel must be lengthened if necessary.
Occasionally, you may want to forge a triconsonantal word whose base and variable vowels do not agree, and cannot be explained away as a-fortification (i.e. burôda, “(very) heavy”) or some other normal process. In such a process, it is recommended to insure that in base form, the two vowels will form a recognized Adûnaic diphthong (e.g. târik, “pillar” à âi). The one exception to the rule is words with a base vowel -i- and secondary vowel -a- (e.g. minal, zidar, “sky” and “poise” respectively).