Metrically speaking, this poem is both simple and complex. Oh great, that's like two for one isn't it? Sure. Let's cover the simple part first. "When We Two Parted" is an example of something called accentual verse, which means that the only metrical rule is that the number of accents, or stresses, per line must remain the same. The number of syllables in the line and the number of unstressed syllables can vary throughout, as long as the number of accented syllables stays the same. We'll call that number the magic number. In "When We Two Parted," that magic number is two.
Let's take a peek at the entire first stanza, just to show you how accentual verse works:
When we two parted
In silence and tears,
Hal broken hearted
To sever for years,
Pale grew thy cheek and cold,
Colder thy kiss;
Truly that hour foretold
Sorrow to this. (1-8)
As you can see, every line but two (we'll get to those in just a sec) has only two stressed syllables. Notice also that, whereas lines 1-5 and 7 have three unstressed syllables, lines 6 and 8 only have two. But hey guess what? Turn that poetry frown upside-down; this is no big deal. As you will recall, this part of the meter (the number of unstressed syllables) is flexible. For the most part, Byron sticks to two stressed syllables, and three unstressed syllables, though he does sometimes use as few as two and as many as four unstressed syllables (remember, gang: flexible).
Now, the two exceptions (lines 5 and 7) in this stanza are also, incidentally, the only two exceptions in the entire poem. They're exceptions because they have three stressed syllables, even though the magic number is supposed to be two. The bright side? At least there are only two. So, was Byron being lazy when he wrote these lines or what? Probably not—he was way too smart for that. Line 5 describes something that, for the speaker, was very traumatic (his friend's sudden transformation into a cold, pale, corpse-like figure) while line 7 narrates a prophecy. Those added syllables emphasize or make us feel more acutely the trauma (line 5) and give the prophecy (line 7) a more ominous tone.
Now, to answer the million dollar question: why use this accentual meter in the first darn place? Well, for one thing, it's a tip of the hat to an older style of poetry. Accentual verse was widely used in an Old English poetry, the most classic example of which is Beowulf. The other thing about accentual verse is that it allows the poet a little bit more freedom than, say, something like iambic pentameter (which is technically accentual syllabic verse). "When We Two Parted" sounds conversational or improvised—like a monologue spouted out at random. This is because the lines are of irregular length (remember: they can be), which makes the poem sound more like an actual sad guy's lament than some super-formal, strictly-metered poem (nobody really talks in iambic pentameter when they're upset, do they?). (Head over to "Sound Check" for more on this poem's… sound.)
Speaking of iambic pentameter, we should talk about this just for a second before we give you the lowdown on some of this poem's other formal elements. We've told you this poem is written in accentual verse, so what about iambs, trochees, and all that business? Well, if you want, you can break down the lines of this poem into small units, or feet, and give them names (this is, essentially, what we do with a poem that is written in a more customary meter like iambic pentameter). Doing that for this poem, which contains no fixed metrical pattern other than the previously mentioned magic number, is tricky, complicated, and (trust us) headache-inducing. Nevertheless, if you want to try, or want to read somebody's opinion about how this should be done, check out this guy's blog right here.
To conclude, we need to talk about a few other little things. Each stanza in this poem contains eight lines. This means that this poem is made up of octets (which is just a fancypants name for an eight-line stanza). Furthermore, each octet in the poem has the following rhyme scheme: ABABCDCD, where each letter represents that line's end rhyme. This means that, in each stanza, the first and third lines rhyme, the second and fourth rhyme, etc. The regularity of the rhyme scheme offsets the semi-irregularity of the meter and injects a sense of certainty in a poem that is plagued by uncertainties. (Will the speaker see his friend again? Will he ever not feel sad when he hears her name? Why did she grow cold when they parted?) It seems that, in this way, the speaker can be sure of one thing: his own ability to craft his feelings into words and hammer those words into a coherent shape. So, hey, he's got that going for him.