The intrinsic nature of the vowels in words, whether long or short, has be learned, or consulted in a dictionary showing this feature: it is not indicated by the spelling.
Additionally, the rules in reading Latin verse are:
1. Vowels intrinsically long remain long.
2. Diphthongs are long.
3. A vowel followed by another vowel and not forming a diphthong is generally short (provided the word is not a Greek borrowing).
4. A short vowel ending a word or followed by single consonant (includes qu, ch, ph and th, but not x) stays short.
5. A short vowel followed by two or more consonants in same word (or single x) becomes long.
6. A short vowel followed by final single consonant in the word, and by another consonant starting the word following, becomes long.
7. Explosive consonants (cpt, gbd) or f are counted as belonging to the next syllable when followed by r or l. This can make the preceding vowel short, or at the poet’s wish, leave the vowel long, though the accent shifts to the following syllable.
Reading also needs to take account of the accent or ictus, which does not necessarily fall on long vowels. The rules are:
1. Two-syllable words are accented on the first syllable.
2. Three-syllable words are accented on the penultimate syllable if long, and the one before that if the penultimate is short.
3. Multisyllable words are accented on the penultimate syllable if long, and the one before that if the penultimate is short.
Expressive reading will slightly modify these rules.