رَأَيْتُ أَخَاكَ.
Translation: I saw your brother.
رَأَيْتُ: past-tense verb; its تُ is the subject pronoun.
أَخَا: direct object, accusative with alif as its sign, and the first term of iḍāfa.
كَ: attached pronoun in the genitive position as the second term of iḍāfa.
Contrast: The brother is now the person seen, not the doer, so the case sign is alif, not wāw.
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Noun Case Classes: Type 4 — The Six Nouns
رَأَيْتُ أَخَاكَ.
Translation: I saw your brother.
رَأَيْتُ: past-tense verb; its تُ is the subject pronoun.
أَخَا: direct object, accusative with alif as its sign, and the first term of iḍāfa.
كَ: attached pronoun in the genitive position as the second term of iḍāfa.
Contrast: The brother is now the person seen, not the doer, so the case sign is alif, not wāw.