For rāʾ, inspect whether it has a vowel or sukūn, the preceding vowel, any following emphatic letter within the word, and whether you are stopping.
A rāʾ with kasrah is generally light, while one with fatḥah or ḍammah is heavy. But a sākin rāʾ cannot always be judged from the preceding kasrah alone.
شِرْعَةً — sākin rāʾ after an original kasrah: light.
قِرْطَاسٌ — ṭāʾ, an emphatic letter, follows within the same word: heavy.
فِرْقٍ — a specific reported variation, not unrestricted choice for other rāʾ occurrences.
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Rāʾ: why one shortcut is insufficient
Comprehensive Revision — Applying Tajwīd Rules
For rāʾ, inspect whether it has a vowel or sukūn, the preceding vowel, any following emphatic letter within the word, and whether you are stopping.
A rāʾ with kasrah is generally light, while one with fatḥah or ḍammah is heavy. But a sākin rāʾ cannot always be judged from the preceding kasrah alone.
شِرْعَةً — sākin rāʾ after an original kasrah: light.
قِرْطَاسٌ — ṭāʾ, an emphatic letter, follows within the same word: heavy.
فِرْقٍ — a specific reported variation, not unrestricted choice for other rāʾ occurrences.