Ayna asks about location; it is not the subject. In tamshī the implicit second-person pronoun is the subject.
Book example: أَيْنَ تَمْشِي؟
Where do you walk?
Ayna: interrogative place adverb built on fatḥ, in accusative adverbial position; tamshī: indicative verb with hidden “you” subject.
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Indeclinable Adverbs — Ghāyāt, Time and Place
Ayna asks about location; it is not the subject. In tamshī the implicit second-person pronoun is the subject.
Book example: أَيْنَ تَمْشِي؟
Where do you walk?
Ayna: interrogative place adverb built on fatḥ, in accusative adverbial position; tamshī: indicative verb with hidden “you” subject.