Gesenius' Hebrew Grammar/77. Relation of the Weak Verbs to one another
a The close relation which exists between some classes of the weak verbs (e.g. between פ״ו and פ״י, ל״א and ל״ה, ע״ע and ע״וּ, ע״ע and ל״ה) appears not only in their similarity or identity of inflexion, or their mutual interchange of certain forms, but especially from the fact that frequently the same root (radix bilittera, see § 30 g) recurs in various weak stems of similar meaning. The meaning accordingly is inherent in the two constant root-consonants, while the third consonant̤, which is weak (and the particular class of weak verbs with it), does not establish any difference in the meaning. Thus from the root דךְ there occur with the same meaning דָּכַךְ, דּוּךְ, דָּכָא to strike, to crush; and from the root נד there are נוּד, נָדַד, נָדָה to flee.
b In this manner the following classes are related in form and meaning:
1. Verbs ע״וּ and ע״ע in which the first and third consonants are the same in both, as being essential to the meaning; e.g. מוּךְ and מָכַךְ to become poor; מוּשׁ and מָשַׁשׁ to feel; נוּד and נָדַד to flee.
c 2. Verbs פ״י and פ״ן; e.g. יָצַב and נָצַב to place, נָקַשׁ and יָקשׁ (yāqōs̆) to lay snares. Moreover, stems belonging to the classes mentioned in 1 (especially ע״וּ) are frequently related also to verbs פ״י and פ״ן, e.g. גּוּר and יָגֹר to fear; טוֹב and יָטַב to be good; נָפַח and פּוּחַ to blow; נָפַץ and פּוּץ to dash to pieces. Verbs פ״א are less frequently connected with these classes, e.g. אָדַשׁ and דּוּשׁ to thresh, &c.
d 3. Verbs ל״א and ל״ה (in which the first two consonants form the real body of the stem) are sometimes related to each other, and sometimes to the above classes. To each other, in דָּכָא and דָּכָה to crush, קָרָא and קָרָה to meet (cf. § 75 nn); to verbs of the other classes, in מָצָה and מָצַץ to suck, דָּחָה and דּוּחַ to thrust, &c.
e 4. Verbs ע״ע and ל״ה, on which cf. Grimm, Journal of Bibl. Lit., 1903, p. 196; e.g. אָנָה and אָנַן to sigh, דָּמָה and דָּמַם to be quiet, חָנָה and חָנַן to incline, כָּלָה and כָּלַל to end, קָלָה and קָלַל to despise, שָׁגָה and שָׁגַג to err, שָׁחָה and שָׁחַח to bend down, שָׁסָה and שָׁסַס to plunder.
f 5. Verbs ע״וּ and ע״ה; e.g. מוּל and מָהַל (New Hebrew; in O.T. only מָהוּל Is 1) to circumcise, מוּר and מָהַר to exchange, נוּר (in מְנוֹרָה a light) and נָהַר to shine; cf. also לְהָטִים secret arts, Ex 7 with לָט secret, from לוּט.