Rule 5: In Form VIII (iftiʿāl), when original wāw or yāʾ occurs next to the form's added tāʾ, the weak radical is changed into tāʾ and the two tāʾs are assimilated.
اِوْتَقَدَ → اِتَّقَدَ — it ignited or flared up.
اِيْتَسَرَ → اِتَّسَرَ — it became easy or available.
The weak radical is not merely deleted: it becomes tāʾ, then merges with the added tāʾ, producing a doubled تّ.
Opening interactive controls...
Rule 5: Assimilation in Form VIII
Weak-Initial Roots: Elision, Substitution, Assimilation and Short Paradigms
Rule 5: In Form VIII (iftiʿāl), when original wāw or yāʾ occurs next to the form's added tāʾ, the weak radical is changed into tāʾ and the two tāʾs are assimilated.
اِوْتَقَدَ → اِتَّقَدَ — it ignited or flared up.
اِيْتَسَرَ → اِتَّسَرَ — it became easy or available.
The weak radical is not merely deleted: it becomes tāʾ, then merges with the added tāʾ, producing a doubled تّ.