Masculine-plural imperfects contain wāw al-jamāʿah. In ordinary prohibitives the indicative nūn falls but the wāw remains; with heavy or light emphatic nūn the ending contracts. Therefore emphasis is not made by simply appending nūn to an unchanged imperfect.
Word / formMeaning / explanation
تَفْعَلُونَ → لَا تَفْعَلُواdo not act (men).
لَا تَفْعَلُنَّdo not act at all (men; heavy nūn).
لَا تَفْعَلُنْcorresponding light-nūn form.
The plural meaning remains, even though the visible ending has undergone a morphological adjustment.