Lā tuhmil fa-tandama has causal fāʾ followed by an implied an, giving the imperfect subjunctive. In tajtahid fa-anta nājiḥun uses fāʾ to link a nominal conditional answer.
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Causal Fāʾ versus Conditional Link
Choosing the Correct Parsing: Meaning, Governance and Context
Lā tuhmil fa-tandama has causal fāʾ followed by an implied an, giving the imperfect subjunctive. In tajtahid fa-anta nājiḥun uses fāʾ to link a nominal conditional answer.