Triconsonantal Clusters are Not allowed in Adûnaic!
Triconsonantal Clusters are when you have three consonants in a row like -zgr-
However, cases like nakkha (“to come”) is NOT a triconsonantal cluster, it is a biconsonantal cluster as the last two consonants are a single sound.
(that is the tricky thing about "clusters"--they're different from a single consonant written with two letters, like English's -sh)
Or like church where there are three letters but only two sounds, R + CH /ʧ/
Circumflexes and Macrons are identical in function in Adûnaic and every other of Tolkien's languages.
The Breve (the rounded line on top of a vowel) indicates a short vowel, like U in the word "cut".
Language has similarities to Kurdish (îdô “now” means the same thing both in Adûnaic and Kurdish).
Masculine endings: -k, -r, -n, -d; in masculine names like Gimilkhâd, Gimilzôr. Possibly -u, -û, or -ô. Attested -l.
Feminine endings: -th, -l, -s, -z; in feminine names like Inzilbêth, Zimraphel. Possibly -i, -î, and -ê. Attested -n.
Historical Periods of Númenor (from https://anthropologyarda.tumblr.com/post/128419467552/historical-periods-of-n%C3%BAmenor)
Foundation Period – The reign of Elros Tar-Minyatur (SA 32-442)
Classical Period – Ta-Vardamir to Tar-Meneldur (SA 442-883)
Colonial Period – Tar-Aldarion to Tar-Minastir; also called the Early Middle Period, the Age of Sail, or the Eldarin Alliance period (SA 883 - 1869)
Imperial Period – Tar-Ciryatan to Tar-Alcarin; during this period the King’s Men and the Elendili factions are established between SA 2221 – 2386. Also includes Anducal’s Uprising and the Reign of the Illegitimate Sovereign, or the Time of the Unseated King; sometimes called the Late Middle Period (SA 1869 – 2737)
Adûnaic Period – Ar-Belzagar to Ar-Gimilzôr; usually divided into early, high and late eras (SA 2737 - 3177)
Revival Era – Tar-Palantir, sometimes referred to as the Days of Restoration by Faithful historians (SA 3177 – 3255)
Avalôiyada Period - Ar-Pharazôn’s reign (SA 3255 – 3319)
Akallabêth Period – Fall of Númenor to the death of Elendil (and sometimes his sons), historiography of the Kingdoms in Exile begins (SA 3319 - 3441)