In يَدْخُلُونَ, the dāl is sākin and belongs to the qalqalah set, so release it with a light, clear bounce without adding a full vowel.
In الْفَتْحُ, the tāʾ is sākin but not a qalqalah letter.
Sukūn alone is not enough: the consonant must also be one of the designated qalqalah letters.
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Yadkhulūna: subtle qalqalah
Al-Kāfirūn and An-Naṣr: Applied Madd, Idghām, Ghunnah and Waqf
In يَدْخُلُونَ, the dāl is sākin and belongs to the qalqalah set, so release it with a light, clear bounce without adding a full vowel.
In الْفَتْحُ, the tāʾ is sākin but not a qalqalah letter.
Sukūn alone is not enough: the consonant must also be one of the designated qalqalah letters.