I should have mentioned that word boundaries aren't respected in determining whether a syllable is open or closed. So the -ν of ἔρωσιν is treated for prosodic purposes as belonging to the following word, ἧς. This presumably reflects the way ancient Greek was spoken--without clear separation between words, like French or Italian.I have σιν heavy. If it is light, why it that?
The traditional way of stating the rule is that a syllable with a short vowel followed by a single consonant is short (light) and if followed by two consonants, long (heavy). It doesn't matter whether or not the consonants belong to the same word.
The basic unit of the dactylic hexameter (the "foot") is a single dactyl, _ v v, but the two short syllables can be replaced by a single long syllable (a spondee). (The term "hexameter" invariably refers to dactylic hexameter, probably the most common verse form in ancient Greek and Latin poetry.)
The hexameter consists of five dactylic feet, with a final spondee (the last syllable is always treated as long, even if it would otherwise be short):
_ v v / _ v v / _ v v / _ v v / _ v v / _ x //
The fifth foot is more often than not a dactyl, not a spondee. This gets to be more and more restrictive over time, until in Latin a spondee in the fifth foot is a special effect, usually a reminiscence of Greek.
A word break (caesura) generally occurs either after the first long syllable of the third foot (a "masculine" caesura) or, if the third foot is a dactyl, not a spondee, after the first short syllable (a "feminine" caesura), dividing the verse into two not quite equal halves; or else there are typically two caesuras, one in the second foot and one in the fourth foot, dividing the verse into three sections.
Word breaks can occur elsewhere in the verse, too, but one of these three patterns of caesuras is more or less obligatory--without them, a verse would not be a well-constructed hexameter.
The interplay of caesuras and the ability to substitute spondees for dactyls are what give life to the hexameter, avoiding monotony. This is especially important, since the hexameter is frequently used for long poems, such as the Iliad and the Odyssey.
Turning to the Philodemus passage,
_ v v / _ v v / _ v σὺν αὐθεντοῦσιν ἄναξιν
[_ v v / _ v v / _ v] v / _ _ / _ v v /_ b //
The part of the presumed verse in brackets is what is lacking. We have what feels like the tail end of the third foot and the three following feet. There is a word break in the third foot before σὺν. This is a feminine caesura, dividing the verse in two sections, more or less in half (though we don't know what the first section was).
in the case of iambic verse, the basic unit is not what we would call an iamb, v _, but rather the iambic metron, x _ v _. Three of these metra make up an iambic trimeter. Two short syllables can be substituted for a long syllable occasionally, but not too often, at least in tragedy; otherwise the iambic feeling would be obliterated. Three of these metra make up an iambic trimeter.