According to Tolkien, “suffixion is used more frequently in inflexion” in Adûnaic (SD/417). This is the case for both noun declension and verb conjugation: the endings for noun cases are all suffixes and most inflections for verb tenses occur at the end of the word.

According to Tolkien (SD/426), Adûnaic suffixes generally end in either a vowel, a dental consonant (t, d, th) or a continuant (s, z, l, r, n, m). Other consonants are less common, though k is not unusual and was apparently a suffix indicating masculinity, as in Ad. mîk “*baby boy”. All of the known Adûnaic suffixes are consistent with this rule.