Gesenius' Hebrew Grammar/41. Variations from the Ordinary Form of the Strong Verb
a The same laws which are normally exhibited in stems with strong (unchangeable) consonants, hold good for all other verbs. Deviations from the model of the strong verb are only modifications due to the special character or weakness of certain consonants, viz.:—
(a) When one of the stem-consonants (or radicals) is a guttural. In this case, however, the variations only occur in the vocalization (according to § 22), not in the consonants. The guttural verbs (§ 62–65) are, therefore, only a variety of the strong verb.
b (b) When a stem-consonant (radical) disappears by assimilation (§ 19 b–f), or when the stem originally, consisted of only two consonants (verbs פ״ן, ע״ע, and ע״וּ, as נָגַשׁ, קַל, קוּם, §§ 66, 67, 72).
c (c) When one of the stem-consonants (radicals) is a weak letter. In this case, through aphaeresis, elision, &c., of the weak consonant, various important deviations from the regular form occur. Cf. § 68 ff. for these verbs, such as יָשַׁב, מָצָא, גָּלָה.
d Taking the old paradigm פָּעַל as a model, it is usual, following the example of the Jewish grammarians, to call the first radical of any stem פ, the second ע, and the third ל. Hence the expressions, verb פ״א for a verb whose first radical is א (primae radicalis [sc. literae] א); ע״ו for mediae radicalis ו; ע״ע for a verb whose second radical is repeated to form a third.