عَلِمْتُ أَنْ سَيَحْضُرُ زَيْدٌ.
I learned for certain that Zayd would attend. In this analysis, an after certainty is lightened from anna; sa-yaḥḍuru remains indicative, not subjunctive.
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An after Certainty or Knowledge
The Subjunctive Imperfect and Its Governors
عَلِمْتُ أَنْ سَيَحْضُرُ زَيْدٌ.
I learned for certain that Zayd would attend. In this analysis, an after certainty is lightened from anna; sa-yaḥḍuru remains indicative, not subjunctive.