Gesenius' Hebrew Grammar/78. Verba Defectiva
78a It often happens, when two kindred weak verbs are in use with the same meaning, that both are defective, i.e. do not occur in all the forms. Since, however, those tenses and forms which are not in use in the one verb are generally supplied by the other, they mutually complete one another, and thus form together, as it were, an entire verb, as in Greek ἔρχομαι, aor. ἦλθον, fut. ἐλεύσομαι, and in Latin fero, tuli, latum, ferre, &c., but with this difference, that in Hebrew the roots of these verbs are almost always closely related.
78b The most common verbs of this kind are—
בּשׁ to be ashamed. Hiphʿîl הֵבִישׁ (inferred from הֱבִישׁ֫וֹתָ), but also הֹבִישׁ, הוֹבִישׁ as if from יבשׁ, on the analogy of verbs פ״ו; also in Is 305 the Qerê requires הֹבִישׁ, where the Kethîbh has הִבְאִישׁ from בָּאַשׁ.
טוֹב to be good. Perfect טוֹב; but imperfect יִיטַב. and Hiphʿîl הֵיטִיב from יָטַב (but cf. הֱטִיבֹ֫תָ 2 K 1030).
יָגׄר to be afraid. Imperfect יָגוּר (from גּוּר).
יָקַץ to awake, only in the imperf. יִיקַץ; for the perfect, the Hiphʿîl הֵקִיץ is used (from קוּץ).
נָפַץ to break in pieces. Imperfect יָפוּץ (from פּוּץ). Imperative פּוּץ. Niphʿal נָפוֹץ. Piʿēl נִפֵּץ (from נָפַץ). Pôlēl פּוֹצֵץ (from פּוּץ). reflexive הִתְפּוֹצֵץ. Hiphʿîl הֵפִיץ. Also פִּצְפֵּץ Jb 1612.
נָצַב (Qal in post-biblical Hebrew, in Aramaic and Arabic) to place, whence (possibly) Niphʿal נִצַּב and Hiph‛îl הִצִיב (see above, § 71); but Hithpaʿēl הִתְיַצֵּב.
שָׁתָה to drink, used in Qal; but in Hiph. הִשְׁקָה to give to drink, from a Qal שָׁקָה which is not used in Hebrew.
On הָלַךְ (יָלַךְ) to go, see above, §69x.
78c Rem. 1. To the same category belong also, to a certain extent, those cases where the tenses or moods not in use in one conjugation, are supplied by forms having the same meaning in other conjugations of the same verb. Thus:
כָּשַׁל to stumble. Perfect from Qal, imperfect from Niphʿal.
נגשׁ to approach, unused in perf. Qal, instead of which Niphʿal נִגַּשׁ is used; but imperfect יִגַּשׁ, imperative גַּשׁ, and infinitive גֶּ֫שֶׁת from Qal only are in use.
נָחָה to lead. Perfect usually נָחָה in Qal, so imperative נְחֵה, but imperfect and infinitive always in Hiphʿîl.
נתך to be poured out. Perfect Niphʿal נִתַּךְ with imperfect Qal יִתַּךְ, but the perfect Qal and imperfect Niphʿal are not in use.
2. The early grammarians often speak of mixed forms (formae mixtae), i.e. forms which unite the supposed character and meaning of two different tenses, genders, or conjugations. Most of the examples adduced are at once set aside by accurate grammatical analysis; some others appear to have arisen from misapprehension and inaccuracy, especially from erroneous views of unusual plene forms. Others, again, are either merely wrong readings or represent an intentional conflation of two different readings.