Senecan Iambic Trimeter

I'll not repeat what others have already said on line about distinguishing short and long vowels in Latin verse and about elision and prodelision. See here and here

Seneca's iambic trimeter closely follows Greek trimeter but it is more regular. It is called trimeter because of three metra. Each metron has two feet like this: x - u -. In this pattern - is long, u is short and x is anceps (either short or long)
Thus the whole line divided into metra is : x - u - | x - u - | x - u -. A long or more rarely an anceps syllable may be replaced with two shorts--this is called metrical "substitution".
Note that every other foot is an iamb or its equivalent in length, namely the tribrach u u u (3 shorts). The first foot in each metron may also be - - (spondee) or u u - (anapaest) or - u u (dactyl) or u u u u (proceleusmatic), which are all one half measure longer than the iamb (u -) and tribrach (u u u).

Here is some iambic trimeter scansion from Seneca's Medea:

| indicates the separation between metra. Notice that this comes before a single consonant (se|d a|lius) even if this puts the final consonant of a word with the following word, but divide after the first consonant or consonant group when multiple consonants come between syllables (mon|strum)

|| indicates a major caesura or pause in the line due to word break and sense

Di coniuga|les || tuque geni|alis tori,
- - u - | - || - u u u | - - u -

Lucina, cus|tos || quaeque domi|turam freta
- - u - | - || - u u u | - - u x {The last syllable here is long because last in line.

Tiphyn nouam |frenare || docu|isti ratem,
- - u - | - - u || u u | - - u -

letumque socer(o) et || regiae stirpi date. {The o in socero elides.
- - u u u | - || - u - | - - u x {The last syllable here is long because last in line.

Note that all these examples follow Porson's Law, perhaps the most well known law of classical metrics, for which see here. It applies to Greek tragic verse (and somewhat to Seneca as he followed Greek practice closely). Basically if the anceps syllable beginning the third metron is long, then it must belong to the same word as the next syllable, unless one of the syllables is a monosyllabic word.