The source says that when hamzat al-waṣl belongs to the definite article, it begins with fatḥah. ٱلَّذِينَ is among the examples of an article-led beginning.
Do not transfer this starting vowel to every verb or other noun. First determine whether the initial element is the article al-.
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Starting with the article al-
Rules of Hamzah — Taḥqīq, Tashīl, Ibdāl and Hamzat al-Waṣl
The source says that when hamzat al-waṣl belongs to the definite article, it begins with fatḥah. ٱلَّذِينَ is among the examples of an article-led beginning.
Do not transfer this starting vowel to every verb or other noun. First determine whether the initial element is the article al-.