The source says kasrah and ḍammah are maʿrūf, not majhūl in Arabic speech. The pedagogical point is that neither should be vague: pronounce kasrah with its distinct mouth setting and ḍammah with clear lip rounding.
Do not collapse «رِ» and «رُ» into a single indistinct vowel; keep the contrast across other consonants as well.
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Kasrah and ḍammah are distinct sounds
Correct Vowel Articulation — Fatḥah, Kasrah, Ḍammah, Ishbāʿ and Sukūn
The source says kasrah and ḍammah are maʿrūf, not majhūl in Arabic speech. The pedagogical point is that neither should be vague: pronounce kasrah with its distinct mouth setting and ḍammah with clear lip rounding.
Do not collapse «رِ» and «رُ» into a single indistinct vowel; keep the contrast across other consonants as well.