604. The Quantity of Final Syllables is as follows.
a. Monosyllables ending in a vowel are long.
1. The attached particles -nĕ, -quĕ,-vĕ, -cĕ, -ptĕ, and rĕ- (rĕd-) are short; sē- (sēd-) and dī-are long.
But re- is often long in rēligiō (relligiō), rētulī (rettulī), rēpulī (reppulī).
b. Nouns and adjectives of one syllable are long.
c. Most monosyllabic Particles are short.
But crās, cūr, ēn, nōn, quīn, sīn with adverbs in c (hīc, hūc, sīc) are long.
d. Final a in words declined by cases is short, except in the Ablative singular of the 1st declension; in all other words final a is long.
eă stellă (Nominative), cum eā stellā (Ablative)
trīgintă, in late use, etc.
e. Final e is short.
Final e is long:
1. In adverbs formed from adjectives of the first and second declension, with others of like form.
(So ferē and fermē)
But it is short in benĕ, malĕ; īnfernĕ, supernĕ.
2. In nouns of the 5th declension.
fidē (also famē)
quārē (quā rē)
3. In Greek neuters plural of the 2nd declension.
4. In the imperative singular of the 2nd conjugation.
However, sometimes it is short.
vidĕ (cf. § 629.b.1)
f. Final i is long.
Final i is common in mihi, tibi, sibi, ibi, ubi; and short in nisĭ, quasĭ, sīcutĭ, cuĭ (when making two syllables), and in Greek Vocatives (Alexĭ).
g. Final o is common, but long in Datives and Ablatives; also in nouns of the 3rd declension. It is almost invariably long in verbs before the time of Ovid.
cedŏ (the imperative)
So sometimes octŏ, īlicŏ, etc., particularly in later writers.
h. Final u is long. Final y is short.
i. Final -as, -es, and -os, are long; final -is, -us, -ys, are short.
1. -as is short in anăs and in Greek plural Accusatives.
2. -es is short in the Nominative of nouns of the 3rd declension (lingual) that have a short vowel in the stem.1
the Present of esse (ĕs, adĕs)
the preposition penĕs
the plural of Greek nouns (hērōĕs, lampadĕs)
3. -os is short in compŏs, impŏs; in the Greek Nominative ending (barbitŏs) and the old Nominative of the 2nd declension (servŏs; later servus).
5. -is is long in plural cases.
omnīs (Accusative plural)
5. -is is long in the verb forms fīs, sīs, vīs (with quīvīs etc.), velīs, mālīs, nōlīs, edīs; in the 2nd person singular Present indicative active in the 4th conjugation (audīs); and sometimes in the forms ending in -eris (Future Perfect indicative or Perfect subjunctive).
6. -us is long in the Genitive singular and Nominative, Accusative, and Vocative plural of the 4th declension, and in nouns of the 3rd declension having ū (long) in the stem.
j. Of other final syllables, those ending in a single consonant are short.
hīc (also hĭc)
The Ablatives illōc, etc.
Certain adverbs in -c (illīc, istūc)
Some Greek nouns (āēr, aethēr, crātēr)