Inspect the word: أَخَاكَ
Non-diminutive? Yes.
Singular? Yes.
In iḍāfa? Yes, to «كَ».
Not added to my-yāʾ? Yes, it is «كَ», not «ي».
All four conditions are met. Thus «أَخَا» is accusative with alif when its role in the sentence calls for accusative case.
Correct sequence: noun type → four conditions → governor and role → case sign.
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Noun Case Classes: Type 4 — The Six Nouns
Inspect the word: أَخَاكَ
Non-diminutive? Yes.
Singular? Yes.
In iḍāfa? Yes, to «كَ».
Not added to my-yāʾ? Yes, it is «كَ», not «ي».
All four conditions are met. Thus «أَخَا» is accusative with alif when its role in the sentence calls for accusative case.
Correct sequence: noun type → four conditions → governor and role → case sign.