In this section فِي is an iḍāfah form of the noun “mouth” meaning my mouth.
But in فِي الْبَيْتِ, fī is the preposition meaning “in the house.”
Identical written shapes do not always have identical grammar: use meaning and context.
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Fī Can Be a Noun Here
Iḍāfah to Yāʾ al-Mutakallim and the Six Nouns
In this section فِي is an iḍāfah form of the noun “mouth” meaning my mouth.
But in فِي الْبَيْتِ, fī is the preposition meaning “in the house.”
Identical written shapes do not always have identical grammar: use meaning and context.