لَيَنْصُرَنَّ — third-person masculine singular:
لَيَنْصُرَنَّ اللَّهُ مَنْ يَنْصُرُهُ
لَتَدْخُلُنَّ — second-person masculine plural:
لَتَدْخُلُنَّ الْمَسْجِدَ الْحَرَامَ إِنْ شَاءَ اللَّهُ
The endings identify different grammatical slots even though both sentences carry emphasis.
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Emphasis in the Imperfect: Light and Heavy Nun
لَيَنْصُرَنَّ — third-person masculine singular:
لَيَنْصُرَنَّ اللَّهُ مَنْ يَنْصُرُهُ
لَتَدْخُلُنَّ — second-person masculine plural:
لَتَدْخُلُنَّ الْمَسْجِدَ الْحَرَامَ إِنْ شَاءَ اللَّهُ
The endings identify different grammatical slots even though both sentences carry emphasis.