When فَمٌ takes these special mudaf forms, its م drops:
فُو الطِّفْلِ — nominative.
فَا الطِّفْلِ — accusative.
فِي الطِّفْلِ — genitive.
Do not confuse فِي الطِّفْلِ here (“the child’s mouth,” in genitive form) with the preposition فِي الْبَيْتِ (“in the house”); context and idafah distinguish them.
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The Five Nouns: Core Case Forms
When فَمٌ takes these special mudaf forms, its م drops:
فُو الطِّفْلِ — nominative.
فَا الطِّفْلِ — accusative.
فِي الطِّفْلِ — genitive.
Do not confuse فِي الطِّفْلِ here (“the child’s mouth,” in genitive form) with the preposition فِي الْبَيْتِ (“in the house”); context and idafah distinguish them.