603. The following are General Rules of Quantity (cf. §§ 9 - 11).
Quantity of Vowels
a. Vowels: A vowel before another vowel or h is short.
1. In the Genitive form -ius, ī is long.
It is, however, sometimes short in verse (§ 113.c).
2. In the Genitive and Dative singular of the 5th declension, e is long between two vowels.
Otherwise, it is usually short.
Note— It was once long in these also.
plēnus fidēī (Ennius, at the end of a hexameter)
A is also long before ī in the old Genitive of the 1st declension.
3. In the conjugation of fīō, i is long except when followed by -er.
fīō, fīēbam, fīam
So also fĭt (§ 606.a.3).
4. In many Greek words the vowel in Latin represents a long vowel or diphthong, and retains its original long quantity.
Note— But many Greek words are more or less Latinized in this respect.
5. In dīus, in ē̆heu usually, and sometimes in Dī̆āna and ō̆he, the first vowel is long.
b. Diphthongs. A Diphthong is long.
Exception—The preposition prae in compounds is generally shortened before a vowel.
praĕ-ustīs (Aen. 7.524)
praĕ-eunte (Aen. 5.186)
c. Contraction. A vowel formed by contraction (crasis) is long.
nīl, from nihil
cōgō for †co-agō
mālō for mā-volō
deinde (two syllables, for de-in-de)
meōs (one syllable, for me-ōs)
Often two syllables are united by Synæresis without contraction: as when părĭĕtĭbŭs is pronounced paryĕtĭbus.
d. A vowel before -ns, -nf, or -gn, is long.
Quantity of Syllables
e. A syllable is long if it contains a long vowel or a diphthong.
f. Position. A syllable is long by position if its vowel, though short, is followed by two consonants or a double consonant.
But if the two consonants are a mute followed by l or r the syllable may be either long or short (common).
alacris or alăcris
patris or pătris
Vowels should be pronounced long or short in accordance with their natural quantity without regard to the length of the syllable by position.
Note 1— The rules of Position do not, in general, apply to final vowels before a word beginning with two consonants.
Note 3— Compounds of iaciō, though written with one i, commonly retain the long vowel of the prepositions with which they are compounded, as if before a consonant, and, if the vowel of the preposition is short, the first syllable is long by position on the principle of § 11.e.
obicis hostī (at the end of a hexameter, Aen. 4.549)
inicit et saltū (at the beginning of a hexameter, Aen. 9.552)
prōice tēla manū (at the beginning of a hexameter, Aen. 6.836)
Later poets sometimes shorten the preposition in trisyllabic forms, and prepositions ending in a vowel are sometimes contracted as if the verb began with a vowel.
(1) cūr an|nōs ŏbĭ|cis (Claud. iv C. H. 264).
(2) reīcĕ că|pellās (Ecl. 3.96, at end).
Note 4— The y or w sound resulting from synæresis has the effect of a consonant in making position
Conversely. when the semivowel becomes a vowel, position is lost.
sĭlŭae for silvae