Compare the original imperfect with the light-nūn form:
● Masculine plural: يَفْعَلُونَ → لَيَفْعَلُنْ — wāw and the inflectional nūn are removed.
● Second-person feminine singular: تَفْعَلِينَ → لَتَفْعَلِنْ — yā and the inflectional nūn are removed.
The worked light form is also shown as لَنَسْفَعَنْ = لَنَسْفَعًا:
كَلَّا لَئِنْ لَمْ يَنْتَهِ لَنَسْفَعًا بِالنَّاصِيَةِ.
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Emphasis in the Imperfect: Light and Heavy Nun
Compare the original imperfect with the light-nūn form:
● Masculine plural: يَفْعَلُونَ → لَيَفْعَلُنْ — wāw and the inflectional nūn are removed.
● Second-person feminine singular: تَفْعَلِينَ → لَتَفْعَلِنْ — yā and the inflectional nūn are removed.
The worked light form is also shown as لَنَسْفَعَنْ = لَنَسْفَعًا:
كَلَّا لَئِنْ لَمْ يَنْتَهِ لَنَسْفَعًا بِالنَّاصِيَةِ.