The book says only certain adverbs are built; not all adverbial nouns.
Teaching example: جَلَسْتُ حَيْثُ جَلَسَ الْمُعَلِّمُ. — I sat where the teacher sat.
Ḥaythu: locative adverb built on ḍamm, in accusative position; the clause j alasa al-muʿallimu is its genitive complement.
The final ḍamma is built, not a nominative case marker.
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The indeclinable noun: definition, rule and eight classes
The book says only certain adverbs are built; not all adverbial nouns.
Teaching example: جَلَسْتُ حَيْثُ جَلَسَ الْمُعَلِّمُ. — I sat where the teacher sat.
Ḥaythu: locative adverb built on ḍamm, in accusative position; the clause j alasa al-muʿallimu is its genitive complement.
The final ḍamma is built, not a nominative case marker.