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.

[*] a. From adjectives of the first and second declensions by changing the characteristic vowel of the stem to -ē: as, cārē, dearly, from cārus, dear (stem cāro-); amīcē, like a friend, from amīcus, friendly (stem amīco-).

[*] Note.--The ending -ē is a relic of an old ablative in -ēd (cf. § 43. N. 1).

[*] b. From adjectives of the third declension by adding -ter to the stem. Stems in nt- (nom. -ns) lose the t-. All others are treated as i-stems:—

1. fortiter, bravely, from fortis (stem forti-), brave.
2. ācriter, eagerly, from ācer (stem ācri-), eager.
3. vigilanter, watchfully, from vigilāns (stem vigilant-).
4. prūdenter, prudently, from prūdēns (stem prūdent-).
5. aliter, otherwise, from alius (old stem ali-).

[*] Note.--This suffix is perhaps the same as -ter in the Greek -τερος and in uter , alter . If so, these adverbs are in origin either neuter accusatives (cf.d) or masculine nominatives.

[*] c. Some adjectives of the first and second declensions have adverbs of both forms (-ē and -ter). Thus dūrus, hard, has both dūrē and dūriter; miser, wretched, has both miserē and miseriter .

[*] d. The neuter accusative of adjectives and pronouns is often used as an adverb: as, multum, much; facilĕ, easily; quid, why.

This is the origin of the ending -ius in the comparative degree of adverbs (§ 218): as, ācrius, more keenly (positive ācriter ); facilius, more easily (positive facilĕ ).

[*] Note.--These adverbs are strictly cognate accusatives (§ 390).

[*] e. The ablative singular neuter or (less commonly) feminine of adjectives, pronouns, and nouns may be used adverbially: as, falsō, falsely; citŏ ,

quickly (with shortened o ); rēctā ( viā ), straight (straightway); crēbrō, frequently; volgō, commonly; fortĕ, by chance; spontĕ, of one's own accord.

[*] Note.--Some adverbs are derived from adjectives not in use: as, abundē, plentifully (as if from †abundus; cf. abundō, abound); saepĕ, often (as if from †saepis, dense, close-packed; cf. saepēs, hedge, and saepiō, hedge in).