خَفْ is the direct masculine singular imperative. It is derived from تَخَافُ with the imperfect prefix removed and jussive contraction of the long medial vowel. In the masculine plural خَافُوا, long ā is retained before the plural wāw.
TypeArabic formMeaning
Masculine singularخَفْfear!
Masculine dualخَافَاyou two, fear!
Masculine pluralخَافُواyou all, fear!
Feminine singularخَافِيyou (female), fear!
The stem changes with number and endings; it is not simply the singular imperative repeated for each position.