Form X normally adds the initial sequence ا-س-ت to a triliteral root. A regular reference pattern is اِسْتَفْعَلَ يَسْتَفْعِلُ اِسْتِفْعَالًا, but a hollow middle consonant causes changes in the observable forms.
اِسْتَعَانَ — يَسْتَعِينُ — اِسْتِعَانَةٌ
اِسْتَعَارَ — يَسْتَعِيرُ — اِسْتِعَارَةٌ
These hollow verbal nouns also end with tāʾ marbūṭah. That ending is part of the attested morphological formation here rather than an arbitrary feminine label.
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Form X: The Istifʿāl Pattern
Derived Hollow Wāw and Yāʾ Verbs: Short Paradigms and Morphological Reasoning
Form X normally adds the initial sequence ا-س-ت to a triliteral root. A regular reference pattern is اِسْتَفْعَلَ يَسْتَفْعِلُ اِسْتِفْعَالًا, but a hollow middle consonant causes changes in the observable forms.
اِسْتَعَانَ — يَسْتَعِينُ — اِسْتِعَانَةٌ
اِسْتَعَارَ — يَسْتَعِيرُ — اِسْتِعَارَةٌ
These hollow verbal nouns also end with tāʾ marbūṭah. That ending is part of the attested morphological formation here rather than an arbitrary feminine label.