Allen and Greenough's New Latin Grammar for Schools and Colleges
J. B. Greenough, G. L. Kittredge, A. A. Howard, Benj. L. D'Ooge, Ed.

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[*] 606. Rules for the Quantity of Derivatives are:—

[*] a. Forms from the same stem have the same quantity: as, ămō , ămāvistī; gĕnus, gĕneris.

Exceptions.—

1. bōs , lār,mās, pār , pēs , sāl,—alsoarbōs,—have a long vowel in the nominative, though the stem-vowel is short (cf. genitive bŏvis etc.).
2. Nouns in -or, genitive -ōris, have the vowel shortened before the final r: as,honŏr. (But this shortening is comparatively late, so that in early Latin these nominatives are often found long.)
3. Verb-forms with vowel originally long regularly shorten it before final m, r, ort: as, amĕm , amĕr , dīcerĕr, amĕt (compare amēmus ), dīcerĕt , audĭt,fĭt.

   [*] Note.--The final syllable in t of the perfect was long in old Latin, but is short in the classic period.

4. A few long stem-syllables are shortened: as,ācer, ăcerbus . Sodē-iĕrō andpē-iĕrō, weakened from iūrō .

[*] b. Forms from the same root often show inherited variations of vowel quantity (see § 17): as, dīcō (cf. maledĭcus ); dūcō (dŭx, dŭcis); fīdō ( perfĭdus ) vōx , vōcis ( vŏcō ); lēx , lēgis ( lĕgō ).

[*] c. Compounds retain the quantity of the words which compose them as, oc-cĭdō ( cădō ), oc-cīdō ( caedō ), in-īquus ( aequus ).

[*] Note.--Greek words compounded with πρό have o short: as, prŏphēta , prŏlŏgus . Some Latin compounds of prō have o short: as, prŏficīscor , prŏfiteor . Compounds with ne vary: as, nĕfās , nĕgō , nĕqueō , nēquam .