Start with the imperfect verb, add لِ, and apply the jussive change at the end.
يَكْتُبُ ← لِيَكْتُبْ
The final بُ becomes بْ: he should write.
أَدْرُسُ ← لِأَدْرُسْ
The speaker says: I should study.
نَدْرُسُ ← لِنَدْرُسْ
The speakers say: we should study.
The lām does not change who performs the action; the verb form still identifies the doer.
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Start with the imperfect verb, add لِ, and apply the jussive change at the end.
يَكْتُبُ ← لِيَكْتُبْ
The final بُ becomes بْ: he should write.
أَدْرُسُ ← لِأَدْرُسْ
The speaker says: I should study.
نَدْرُسُ ← لِنَدْرُسْ
The speakers say: we should study.
The lām does not change who performs the action; the verb form still identifies the doer.