هَذِهِ عَصَايَ — This is my staff: predicate, nominative with estimated ḍammah on the alif.
أَخَذْتُ عَصَايَ — I took my staff: direct object, accusative with estimated fatḥah.
اتَّكَأْتُ عَلَى عَصَايَ — I leaned on my staff: governed by ʿalā, genitive with estimated kasrah.
The final alif cannot carry those overt case vowels, so the endings are estimated.
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Iḍāfah to Yāʾ al-Mutakallim and the Six Nouns
هَذِهِ عَصَايَ — This is my staff: predicate, nominative with estimated ḍammah on the alif.
أَخَذْتُ عَصَايَ — I took my staff: direct object, accusative with estimated fatḥah.
اتَّكَأْتُ عَلَى عَصَايَ — I leaned on my staff: governed by ʿalā, genitive with estimated kasrah.
The final alif cannot carry those overt case vowels, so the endings are estimated.