Both lām al-amr and prohibitive lā can govern a jussive imperfect, yet their meanings oppose each other. The first demands performance; the second demands abstention. A change in the initial particle may reverse the instruction.
Word / formMeaning / explanation
لِيَكْتُبْlet him write.
لَا يَكْتُبْlet him not write.
لِتَكْتُبْlet you write (lām al-amr construction).
لَا تَكْتُبْdo not write.
The final sukūn may be shared; it is the initial particle and intended meaning that decide whether the clause is affirmative or prohibitive.