Meter in Latin poetry is based on combinations of long and short syllables.

A syllable is long or short depending on its vowel which is said to be either long by nature or long by position. 

Eight Basic Rules for Scansion of Latin Poetry into Long and Short Syllables

1. Vowels may be long by position, that is, they are long because they are followed by two or more consonants, or the double consonant x; these need not be in the same word.

2. All dipthongs, or single vowels which are contracted dipthongs, are long. (ae, oe, etc.)

3. A vowel before another vowel or before ‘h’ is short.

4. Some combinations of letters function as one sound: qu, gu, su sound like qw, gw, sw and are sonsidered on ‘sound’ or ‘letter’ in Latin poetry.

5. Vowels are long by position when they occur before 2 consonants, even if they are not in the same word.

6. Vowels at the end of one word may be cut off if there is a vowel of the letter h at the beginning of the next word. The final -um before est is often elided to -mst.

7. Vowels may short or long by position when certain combinations of consonants function as one sound and are therefore counted as one: combinations with ‘l’ and ‘r’. br, tr. Greek words such as names with ‘m’ and ‘n’ follow this rule in Latin poetry.

8. Vowels long by nature may be found so marked in the dictionary. There are also a number of long vowels within the various inflected endings of the nouns and verbs, such as the lone a, e and I of the infinitives of the 1st, 2nd and 4th conjugations respectively, and the a ablative ending of the 1st declension. Turn to the back of any grammar book or many of the small dictionaries and look at the charts of forms; the long syllables will be so marked. 

Exercises.

There are six feet in a line. A foot may be composed of two (long) or three (long, short, short) syllables. The last foot, however is always 2 syllables and either a trochee (long-short) or a spondee (long-long).

How to scan a line of dactylic hexameter poetry in Latin.

To begin with, mark as ‘long -doubtful’ the last 2 syllables. Then:

Mark all the long vowels following rule 1. (vowels are long before 2 consonants or x)

Mark all the dipthongs long following rule 2. (ae, au, ei, oe, ui, sometimes eu)

Mark as short all the vowels which occur before h or before another vowel in the same word, following rule 3.

Mark all vowels short which occur before qu, gu, su following rule 4 unless, of course, they occur in combination with yet another consonant and are long following rule 1.

Mark all vowels long which occur before 2 consonants even if the consonants are not in the same word, following rule 5.

Divide your marked syllables into 6 feet following the scheme (and sometimes you can fill in the syllables by process of elimination):

This may complete your task but if you have any extra syllables left, hunt for possible elisions, that is, a final syllable ending in a vowel may be ‘cut off’ or joined with the following word if the the word begins with a vowel or with h, following rule 6:

Still doesn’t do it? Look for consonant combinations with l and r which are pronunced together as one sound (tr, pr for example). Vowels before these sounds may be either short or long as required by the meter, following rule 7.

Still more? Turn to the back of any standard grammar book or most small dictionaries and look at the syllables marked long in you paradigms, following rule 8.

Exercise 1.

Arma virumque cano, Troiae qui primus ab oris
Italiam, fato profugus, Laviniaque venit
litora, multum ille et terris iactatus et alto
vi superum saevae memorem Iunonis ob iram;
multa quoque et bello passus, dum conderet urbem,
inferretque deos Latio, genus unde Latinum,
Albanique patres, atque altae moenia Romae.
Musa, mihi causas memora, quo numine laeso,
quidve dolens, regina deum tot volvere casus
insignem pietate virum, tot adire labores
impulerit. Tantaene animis caelestibus irae?

Exercise 2. Try a storm,

Haec ubi dicta, cavum conversa cuspide montem
impulit in latus: ac venti, velut agmine facto,
qua data porta, ruunt et terras turbine perflant.
Incubuere mari, totumque a sedibus imis una
Eurusque Notusque ruunt creberque procellis
Africus, et vastos volvunt ad litora fluctus.
Insequitur clamorque virum stridorque rudentum.
Eripiunt subito nubes caelumque diemque
Teucrorum ex oculis; ponto nox incubat atra.
Iutonuere poli, et crebris micat ignibus aether,
praesentemque viris intentant omnia mortem.