The source identifies three weak letters:
ا — Alif
و — Waaw
ي — Yaa
For the rule in this level, pay special attention when a Mudhaari' verb ends in a weak letter.
If لَمْ makes that ending jussive, the final weak letter is deleted instead of simply receiving a visible Sukoon.
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What Are the Weak Letters?
Rule of Past Absolute Negation with "Lam" (لَمْ) & Weak Letters
The source identifies three weak letters:
ا — Alif
و — Waaw
ي — Yaa
For the rule in this level, pay special attention when a Mudhaari' verb ends in a weak letter.
If لَمْ makes that ending jussive, the final weak letter is deleted instead of simply receiving a visible Sukoon.