وَلَهُ حَرْفٌ وَاحِدٌ وَهُوَ «لَا» الْجَازِمَةُ فِي نَحْوِ قَوْلِكَ: لَا تَفْعَلْ
Its particle is the jussive lā, as in “do not do it.”
The conventional marker of prohibition is lā al-nāhiyah al-jāzimah. It governs a present-tense verb in the jussive: tafʿalu becomes lā tafʿal, with final sukūn on a sound-final singular verb.
Its function is not merely to state that an act has not happened. It prohibits the addressee from performing it. Recognizing both mood and meaning is essential in parsing such sentences.