hi, have a look at an explanation of latin syllables, which should hopefully answer your qn. see e.g. bennett s4:http://www.archive.org/stream/newlating ... 8/mode/2up
in s4 see (3) (which deals with the situation of two consonants together) and then (4) (which deals with exceptions to the rule, i.e. where two consonants together do not always make the preceding syllable heavy).
to give some e.g.s from the beginning of the aeneid:
line 1. ARMA VIRVMQVE CANO...
ARMA falls into s4(3) in bennett's grammar linked above: the first syllable is AR and so it is heavy and so scans long.
line 7. ALBANIQVE PATRES
PATRES here falls into s4(4) in bennett's grammar linked above: the first syllable is PA (both the T and the S get pushed to the beginning of the next syllable), and so it is light (because the vowel is short and there's no consonant at the end of the syllable) and so scans short - although iin other places, syllables like this can scan long in both grk and latin...
by the way, in both grk and latin metrics you'll see that the current approach tends to be to describe syllables as heavy or light rather than long or short. this is to avoid confusion with vowel length. you can see more on this in modern explanations of meter, phonetics etc.
adrian, why do you say "In "amatum", correpta et "a-" et "-ma-" syllaba, longa "-tum