لَا تَفْعَلْ
Do not do this.
Prohibitive lā governs the jussive and seeks restraint. Negative lā, by contrast, can negate a proposition without placing the imperfect in the jussive.
Compare lā yaktubu Zaydun (“Zayd does not write”) with lā taktub (“do not write”). Identical particle spelling is not sufficient: mood, syntax and discourse purpose decide whether the utterance is a report or a prohibition.
ExpressionType of lāVerbForce
Lā taktubProhibitiveJussiveForbid writing
Lā yaktubu ZaydunNegativeIndicativeReport non-writing