The source associates kasrah with inkhifāḍ al-fam, a change in mouth configuration distinct from fatḥah. Without the proper setting, kasrah tends toward fatḥah.
Practice: say «بِ، تِ، دِ» without rounding the lips as for ḍammah. Keep the vowel short: «بِ» contains no long yāʾ. Compare it with «بَ» and «بُ».
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Kasrah: the lowered/flattened mouth position
Correct Vowel Articulation — Fatḥah, Kasrah, Ḍammah, Ishbāʿ and Sukūn
The source associates kasrah with inkhifāḍ al-fam, a change in mouth configuration distinct from fatḥah. Without the proper setting, kasrah tends toward fatḥah.
Practice: say «بِ، تِ، دِ» without rounding the lips as for ḍammah. Keep the vowel short: «بِ» contains no long yāʾ. Compare it with «بَ» and «بُ».