وَاعْلَمْ: أَنَّهُ إِذَا أُضِيفَتِ الظُّرُوفُ إِلَى الْجُمْلَةِ، أَوْ إِلَى «إِذْ» جَازَ بِنَاؤُهَا عَلَى الْفَتْحِ.
Translation: When adverbs are annexed to a clause or to idh, building them on fatḥ can be permitted.
The closing rule allows building on fatḥ for certain adverbs annexed to a clause or idh. This concerns specified constructions, not a blanket rule for every adverb.