The same particle can appear in two different uses. Look at the verb ending to tell them apart.
MeaningFormUse
he does not helpلَا يَنْصُرُNegation; no Jazm
let him not helpلَا يَنْصُرْProhibition; Jazm
you do not helpلَا تَنْصُرُوْنَNegation; Noon remains
do not helpلَا تَنْصُرُوْاProhibition; Noon drops
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The Prohibitive Verb (Fi'l Nahi) - Concept & Construction
The same particle can appear in two different uses. Look at the verb ending to tell them apart.
MeaningFormUse
he does not helpلَا يَنْصُرُNegation; no Jazm
let him not helpلَا يَنْصُرْProhibition; Jazm
you do not helpلَا تَنْصُرُوْنَNegation; Noon remains
do not helpلَا تَنْصُرُوْاProhibition; Noon drops