A verb translated “fall down” (SD/439). This verb has more conjugations than any other attested Adûnaic verb, and is therefore useful for studying the Adûnaic verb system. Excluding the first draft of the Lament of Akallabêth, it appears in four forms: ukallaba “[he] fell” (SD/429), hikallaba “she fell (down)” (SD/427), hikalba “she fell” (VT24/12) and yakalubim “lean over” (SD/251). Andreas Moehn suggested (LGtAG, EotAL/KAL’B) the last of these could be a different verb kalub- “to lean”, but I think it is likelier to be a form of kalab-, as suggested by Carl Hostetter and Patrick Wynne (VSH/26-27).
Various verbal prefixes and suffixes can be extracted by comparison to other forms: the pronominal prefixes u- “he”, hi- “she” and ya- “they (neuter)”, as well as the plural verb suffix -m. Removing these elements, we have three distinct forms: kallaba “fell”, kalba “fell” and kalubi “lean over”. These likely represent distinct verb tenses. The question is: which ones?
Carl Hostetter and Patrick Wynne suggested (VSH/26-27) that kalubi is most likely the continuative-present with the literal sense “are falling over”, a conclusion with which most other authors agree (AL/Adûnaic, NBA/14). Andreas Moehn agreed on this verb tense (LGtAG, EotAL/KAL’B), but suggested that it is from the verb kalub-* “to lean” instead.
The two remaining forms kalba and kallaba both have the gloss “fell”. This ambiguity is because the Adûnaic aorist tense could used to describe the past in a narrative (SD/439). Most authors agree that these forms are the Adûnaic aorist and past tenses respectively, though Carl Hostetter and Patrick Wynne suggested that kallaba is the continuative-past instead (VSH/27-28). See the entry on the Adûnaic past tense for further discussion.
Finally, there is form appearing only in the draft versions of the Lament: akallabi “fell in ruin”. I believe this is an example of draft-perfect tense; see that entry for further discussion.