Compare the feminine addressee:
تَرْمِينَ — لَمْ تَرْمِي — اِرْمِي
Translation: you (f.) throw — you did not throw — throw!
The indicative nūn is lost, while the addressee yāʾ stays.
Now compare the weak-final singular masculine pattern:
يَرْمِي — لَمْ يَرْمِ — اِرْمِ
Translation: he throws — he did not throw — throw! (m. sing.)
Here the weak final root letter itself is deleted in the jussive. Similar-looking changes can have different grammatical causes.
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Weak-Final Rules in the Addressee Forms
Full Conjugation of the Yāʾ-Final Verb رَمَى يَرْمِي
Compare the feminine addressee:
تَرْمِينَ — لَمْ تَرْمِي — اِرْمِي
Translation: you (f.) throw — you did not throw — throw!
The indicative nūn is lost, while the addressee yāʾ stays.
Now compare the weak-final singular masculine pattern:
يَرْمِي — لَمْ يَرْمِ — اِرْمِ
Translation: he throws — he did not throw — throw! (m. sing.)
Here the weak final root letter itself is deleted in the jussive. Similar-looking changes can have different grammatical causes.