In the 5 Dhammah-ending positions, لَمْ and لَنْ look clearly different:
لَمْ يَفْعُلْ
Sukoon at the end.
لَنْ يَفْعُلَ
Fathah at the end.
But in the 7 Noon-dropping positions and the 2 feminine plural positions, their visible endings can look the same.
So the particle itself is important for recognizing the grammatical state.
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Lam and Lan: Where Do They Look Different?
Conjugating Past Absolute Negation with "Lam" (Active & Passive)
In the 5 Dhammah-ending positions, لَمْ and لَنْ look clearly different:
لَمْ يَفْعُلْ
Sukoon at the end.
لَنْ يَفْعُلَ
Fathah at the end.
But in the 7 Noon-dropping positions and the 2 feminine plural positions, their visible endings can look the same.
So the particle itself is important for recognizing the grammatical state.