OK, iambics. Here’s what you need to know. Most if not all of it you evidently know already, but to run through it:
The Greek iambic trimeter is basically
x-v-x/-v-x-v-
where x means the syllable is free to be either short or long (it’s called “anceps”). There’s only three places in the line where there has to be a short syllable.
The meter is taken over by Roman poets such as Seneca in his tragedies. These will provide you with good examples.
Catullus even wrote a couple of poems in “pure” iambics, stricter than the Greek form, with alternation of exclusively short (not anceps) and long syllables:
#4 Phasellus ille, quem videtis, hospites, etc.
#27 Quis hoc potest videre, quis potest pati? etc.
You should be able to read these metrically without difficulty, and I suggest you start by doing just that. (But don’t bother trying to compose iambics like these yourself: you’ll fail. Catullus is showing off his virtuosity.)
The regular trimeter (both Greek and Latin) allows occasional substitutions: some of the longs can be “resolved” into two shorts, as can some of the anceps syllables too. We won’t go into that unless you want to.
The meters of early Roman comedy—Plautus and Terence—are quite a different kettle of fish. It’s these that’ll make you cry.
Most important, nearly all the “short” positions can be either long or short (if you follow me). So instead of an iambic trimeter (each metron being x-v-) you get the iambic senarius (6-footer). The basic form (I stress basic) is
x-x-x/-x-x-v-.
The caesura gives the line a bit of shape, and the penult has to be kept short. (You lose your “3rd-syllable-short” rule.)
The line admits of many kinds of variation, and there are all sorts of weird and wacky phonological phenomena to complicate the scansion of the verses. (They’re not really weird at all, of course, just reflexes of how the Latin language behaved in real life. Despite being metrical, they’re our best guide to how Latin was really spoken.)
But let’s forget about them, and go straight on to the iambic “septenarius” (so-called). This is what our Latin translation of “Miss Bailey” uses, in relatively straightforward form.
It’s basically
x-x-x-v-/x-x-v- -
(You’ll notice it doesn’t actually have 7 feet, as the name implies, but more like 7 and a half. It’s really a catalectic octonarius.)
Note (1) the midline break (contrasting with the senarius’ mid-foot caesura) and (2) the obligatory short in the closing cadence of each of its two parts. These help give the line definition.
So now you can have another go at Miss Bailey.
Seduxit Miles virginem, receptus in hybernis
SeDUXit MIes VIRginEM,
reCEPtus IN hyBERnis
I’ve put the longs in caps. You’ll notice most of them coincide with where the word accents naturally fall. (I’ve put those in bold.) So just read the line as if it were prose, and you should find it practically scans itself!
(Hybernis will be alternative spelling of hibernis. From a Londoner’s perspective I guess Halifax, in Yorkshire, is the frozen North.) He treats the h- as if it were a consonant, contrary to ordinary Latin practice.
Quae laqueo praecipitem / se contulit Avernis.
laqu- is long.
praecipitem you scan correctly, with –cip- short. (Catullus 105!) But the Archdeacon who composed this evidently didn’t know that: here the –cip- has to be scanned long. As does the –it of contulit. Little wonder you were thrown off! But if you have the meter fixed in your head and make the words fit it, you should be able to sail right through his metrical errors, merely wincing if you notice them. I should not have thrown these faulty septenarii at you.
So I hope this lessens your difficulties. Just stay away from Plautus and Terence (at least for now), and you’ll be fine.
—Oh, your trimeter. (Trimeter as distinct from senarius, I take it.)
Tu dicis autem te bene tuba canere sic.
I have difficulty reading this as a trimeter. You’re playing fast and loose with resolutions. Up to caesura is fine, then things go a bit haywire. bene tu- gives you a split resolution (-be tu- substituting for a long), which throws the whole line out, and to make matters worse you follow that up with a fifth-foot dactyl. The feel of the meter is quite lost, I’m afraid. I think you’re working too mechanically. If you must have resolutions, you must keep them clean and clear. Seneca would make a good model. But perhaps you should really get on top of the dactylic meters first.
(But hey, poets like to make things difficult for themselves: witness the second half of the pentameter.)