Rule 4: If the omitted medial radical is wāw but the simple perfect originally has kasrah on its middle radical, the first radical receives kasrah after the weak radical drops.
خَوِفْنَ → خِفْنَ — the women feared.
خَوِفْتُ → خِفْتُ — I feared.
Therefore kasrah in خِفْتُ does not prove a yāʾ root. Its root is خ و ف, and the vowel reflects the original faʿila-type perfect.
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Rule 4: Why خِفْتُ Has Kasrah
Hollow Verbs: Seven Iʿlāl Rules and Three Short Paradigms
Rule 4: If the omitted medial radical is wāw but the simple perfect originally has kasrah on its middle radical, the first radical receives kasrah after the weak radical drops.
خَوِفْنَ → خِفْنَ — the women feared.
خَوِفْتُ → خِفْتُ — I feared.
Therefore kasrah in خِفْتُ does not prove a yāʾ root. Its root is خ و ف, and the vowel reflects the original faʿila-type perfect.