In the underlying form of خَافَ, a mobile medial wāw after fatḥah becomes alif. Before consonantal endings, however, the long alif cannot remain, so it disappears and the first consonant takes kasrah.
خَوِفَ → خَافَ
خِفْتُ — I feared.
خِفْنَ — they (females) feared.
A fabricated `خَافْتُ` is not the actual conjugation. The correct contracted form is خِفْتُ.
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Why Khāfa Becomes Khiftu
Complete Conjugation of Khāfa–Yakhāfu: Hollow Wāw from the Samiʿa Class
In the underlying form of خَافَ, a mobile medial wāw after fatḥah becomes alif. Before consonantal endings, however, the long alif cannot remain, so it disappears and the first consonant takes kasrah.
خَوِفَ → خَافَ
خِفْتُ — I feared.
خِفْنَ — they (females) feared.
A fabricated `خَافْتُ` is not the actual conjugation. The correct contracted form is خِفْتُ.