After assimilation, the beginning contains a silent consonant.
Arabic pronunciation does not begin with a silent consonant, so a connecting Hamzah is added at the front.
In this lesson it is pronounced with Kasrah: اِـ.
This is why the assimilated forms begin with forms such as اِثَّـ and اِطَّـ.
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Why Hamzat-ul-Wasl Is Needed
Derived Trilateral - Bab 8: If'aal & Bab 9: If'il (Assimilation Doors)
After assimilation, the beginning contains a silent consonant.
Arabic pronunciation does not begin with a silent consonant, so a connecting Hamzah is added at the front.
In this lesson it is pronounced with Kasrah: اِـ.
This is why the assimilated forms begin with forms such as اِثَّـ and اِطَّـ.