Replacing the hamzah
◆الإبدال
al-ibdāl
The hamzah is dropped and replaced outright by a letter of prolongation matching the preceding vowel — ā after fatḥah, ū after ḍammah, ī after kasrah. So يُؤْمِنُونَ becomes يُومِنُونَ.
Example
يُومِنُونَ
al-Baqarah 2:3
al-Sūsī and Abū Jaʿfar replace the hamzah; Ḥafṣ keeps it.