The heavy emphatic nūn «نَّ» increases the force of a prohibition. An ordinary prohibitive asks that the act not happen; the emphatic form puts special stress on that restriction. The textbook provides its heavy-nūn forms for all fourteen persons.
Word / formMeaning / explanation
لَا تَفْعَلْdo not do it.
لَا تَفْعَلَنَّdo not do it at all!
لَا يَفْعَلْlet him not do it.
لَا يَفْعَلَنَّhe must not do it.
The heavy nūn is a grammatical emphasis marker, not a permanent consonant of the root, and the preceding ending may change with person and number.