The source’s general rule has two steps: take the appropriate active or passive imperfect, add lā at the beginning, and apply the jussive rules. The method serves all fourteen positions—third, second and first person.
Word / formMeaning / explanation
يَفْعَلُ ← لَا يَفْعَلْlet him not do it.
تَفْعَلُ ← لَا تَفْعَلْdo not do it (male).
نَفْعَلُ ← لَا نَفْعَلْlet us not do it.
The root consonants remain in place; the inserted prohibitive and the inflectional ending carry the primary grammatical change.