Do not conclude that every appearance of أَبٌ must be أَبُو. Compare:
هَذَا أَبٌ — this is a father: not mudaf, so the ordinary form remains.
جَاءَ أَبُو زَيْدٍ — Zayd’s father came: singular and mudaf, so the special nominative form أَبُو appears.
Only ذُو is stated here as always mudaf. The other four take these special singular forms when they are singular and mudaf. Their dual and plural forms follow their own patterns.
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The Five Nouns: Core Case Forms
Do not conclude that every appearance of أَبٌ must be أَبُو. Compare:
هَذَا أَبٌ — this is a father: not mudaf, so the ordinary form remains.
جَاءَ أَبُو زَيْدٍ — Zayd’s father came: singular and mudaf, so the special nominative form أَبُو appears.
Only ذُو is stated here as always mudaf. The other four take these special singular forms when they are singular and mudaf. Their dual and plural forms follow their own patterns.