Anta is the subject of the nominal clause. Kayfa is an indeclinable interrogative expressing state, syntactically in the predicate position, not a place adverb.
Book example: كَيْفَ أَنْتَ؟
How are you?
Kayfa: indeclinable interrogative in the nominative predicate position; anta: independent pronoun in the nominative subject position.
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Indeclinable Adverbs — Ghāyāt, Time and Place
Anta is the subject of the nominal clause. Kayfa is an indeclinable interrogative expressing state, syntactically in the predicate position, not a place adverb.
Book example: كَيْفَ أَنْتَ؟
How are you?
Kayfa: indeclinable interrogative in the nominative predicate position; anta: independent pronoun in the nominative subject position.