Always identify the particle before analysing the form:
FormSubjunctive after lanJussive after lam
3m.sg. activeلَنْ يَرْمِيَلَمْ يَرْمِ
3m.sg. passiveلَنْ يُرْمَىلَمْ يُرْمَ
3m.du. activeلَنْ يَرْمِيَالَمْ يَرْمِيَا
3m.pl. activeلَنْ يَرْمُوالَمْ يَرْمُوا
2f.sg. activeلَنْ تَرْمِيلَمْ تَرْمِي
Both particles remove the indicative nūn of the five verbs. Yet lan governs the subjunctive while lam governs the jussive; the underlying grammatical cause differs even when an output form looks alike.
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Full Conjugation of the Yāʾ-Final Verb رَمَى يَرْمِي
Always identify the particle before analysing the form:
FormSubjunctive after lanJussive after lam
3m.sg. activeلَنْ يَرْمِيَلَمْ يَرْمِ
3m.sg. passiveلَنْ يُرْمَىلَمْ يُرْمَ
3m.du. activeلَنْ يَرْمِيَالَمْ يَرْمِيَا
3m.pl. activeلَنْ يَرْمُوالَمْ يَرْمُوا
2f.sg. activeلَنْ تَرْمِيلَمْ تَرْمِي
Both particles remove the indicative nūn of the five verbs. Yet lan governs the subjunctive while lam governs the jussive; the underlying grammatical cause differs even when an output form looks alike.