أَيْنَ تَذْهَبُ؟
Where are you going?
Ayna requests a place or location. “To the mosque” or “to the market” is an appropriate answer; “yesterday” or “five” misses what the question asks.
أَيْنَ الْكِتَابُ؟
Where is the book?
The unknown is the book's location, not one of its properties.
Meaning contrast: “Ayna tadhhab?” seeks destination; “matā tadhhab?” asks about time; “kayfa tadhhab?” asks about manner. The same verb may be involved, but each form requests a different piece of knowledge.