Rule 1: 1. أَبٌ and أَخٌ before a genitive
When أَبٌ father or أَخٌ brother is the first noun of an iḍāfa, or carries any suffix except ـِي my, its case vowel is written long: أَبُو in the nominative, أَبَا in the accusative, أَبِي in the genitive. The same holds for أَخُو, أَخَا, أَخِي. With the suffix my all three cases look alike: أَبِي my father, أَخِي my brother.
أَبُو صَدِيقِي مُعَلِّمٌ.
abū ṣadīqī muʿallimun
My friend's father is a teacher.
Nominative: أَبُو.
رَأَيْتُ أَبَا صَدِيقِي.
raʾaytu abā ṣadīqī
I saw my friend's father.
Accusative: أَبَا.
ذَهَبْتُ إِلَى أَبِي صَدِيقِي.
dhahabtu ilā abī ṣadīqī
I went to my friend's father.
Genitive: أَبِي.
أَخُو الْمُعَلِّمِ فِي الْحَدِيقَةِ.
akhu-l-muʿallimi fi-l-ḥadīqati
The teacher's brother is in the garden.
سَأَلْتُ أَخَا الْمُعَلِّمِ.
saʾaltu akha-l-muʿallimi
I asked the teacher's brother.
هٰذَا قَفَصُ أَخِي الْحَارِسِ.
hādhā qafaṣu akhi-l-ḥārisi
This is the cage of the keeper's brother.