Correct imperative derivation requires more than changing one end letter. Identify the imperfect person, its verb form and the middle-root vowel, then apply either the special direct-active second-person rule or the general lām al-amr rule.
Word / formMeaning / explanation
تَسْمَعُ → اِسْمَعْ → اِسْمَعَا → اِسْمَعُواlisten (one male), you two listen, you men listen.
تَكْتُبُ → اُكْتُبْ → اُكْتُبِي → اُكْتُبْنَwrite (one male), one female, women plural.
يَكْتُبُ → لِيَكْتُبْlet him write.
The final example refers to the third person, so it uses imperative lām rather than the connecting hamzah of the direct active command.