«زيد في الدار»
Zayd is in the house.
The preposition fī conveys a locative relation. The sentence does not call Zayd a house; it locates him in one.
Subject: Zayd.
What is expressed: being in the house.
Mediator: fī, which carries the locative relation.
Replacing it with “Zayd is a house” destroys the intended claim. This contrast makes the role of an intermediary in derivative predication clear.