PurposeCentral meaningKey contextual clue
DuʿāʾHumble pleaNeed and supplication
IltimāsPeer-to-peer requestEquality and cooperation
TamannīExpressed wishDesired but unattainable or unlikely event
TahdīdWarning of consequencesReproach and threat of harm
All four may retain prohibitive grammar while expressing distinct intentions.
Working principle: identify the form through morphology and syntax, then identify its rhetorical function through the demands of the situation.