For madd alif, the book makes the quality of the preceding consonant decisive: an emphatic consonant gives the lengthened vowel an emphatic colouring, and a light consonant leaves it light.
Compare طَالَ and قَالَ with كَانَ. Keep the natural long vowel in all of them; the distinction is the quality carried over from the preceding consonant, not an extra consonant or a separate vowel added to alif.
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Alif follows the preceding sound
Tafkhīm and Tarqīq — Alif, Divine-Name Lām and Rāʾ
For madd alif, the book makes the quality of the preceding consonant decisive: an emphatic consonant gives the lengthened vowel an emphatic colouring, and a light consonant leaves it light.
Compare طَالَ and قَالَ with كَانَ. Keep the natural long vowel in all of them; the distinction is the quality carried over from the preceding consonant, not an extra consonant or a separate vowel added to alif.