Gesenius' Hebrew Grammar/25. Unchangeable Vowels
25a What vowels in Hebrew are unchangeable, i.e. are not liable to attenuation (to Šewâ), modification, lengthening, or shortening, can be known with certainty only from the nature of the grammatical forms, and in some cases by comparison with Arabic (cf. §1m). This hems good especially of the essentially long vowels, i.e. those long by nature or contraction, as distinguished from those which are only lengthened rhythmically, i.e. on account of the special laws which in Hebrew regulate the tone and the formation of syllables. The latter, when a change takes place in the position of the tone or in the division of syllables, readily become short again, or are reduced to a mere vocal Šewâ.
25b 1. The essentially long and consequently, as a rule (but cf. §26p, §27n, o), unchangeable vowels of the second and third class, î, ê, û, ô, can often be recognized by means of the vowel letters which accompany them (־ִי, ־ֵי, וּ, וֹ); e.g. יֵיטִיב he does well, חֵיכָל palace, גְּבוּל boundary, קוֹל voice. The defective writing (§8i) is indeed common enough, e.g. יֵיטִב and יֵטִיב for יֵיטִיב; גְּבֻל for נְבוּל; קֹל for קוֹל, but this is merely an orthographic licence and has no influence on the quantity of the vowel; the û in גְּבֻל is just as necessarily long, as in גְּבוּל.
As an exception, a merely tone-long vowel of both these classes is sometimes written fully, e.g. יִקְטוֹל for יִקְטֹל.
25c 2. The essentially or naturally long â (Qameṣ impure), unless it has become ô (cf. §9q), has as a rule in Hebrew no representative among the consonants, while in Arabic it is regularly indicated by א; on the few instances of this kind in Hebrew, cf. §9b, §23g. The naturally long â and the merely tone-long ā therefore can only be distinguished by an accurate knowledge of the forms. 25d 3. Short vowels in closed syllables (§26b), which are not final, are as a rule unchangeable, e.g. מַלְבּוּשׁ garment, מִדְבָּר wilderness, מַמְלָכָה kingdom; similarly, short vowels in sharpened syllables, i.e. before Dageš forte, e.g. גַּנָּב thief.
25e 4. Finally, those long vowels are unchangeable which, owing to the omission of the strengthening in a guttural or ר, have arisen by lengthening from the corresponding short vowels, and now stand in an open syllable, e.g. מֵאֵן for mĭʾʾēn; בֹּרַךְ for burrakh.
- By vocales impurae the older grammarians meant vowels properly followed by a vowel letter. Thus כְּתָב kethâbh was regarded as merely by a licence for כְּתָאב, &c.