On the page, Mariana looks, well, pretty much exactly like what you'd expect a poem to look like. We have seven stanzas, each with twelve lines. And, within each of the stanzas, we have some rhyming. For example:
Her tears fell with the dews at even; A
Her tears fell ere the dews were dried; B
She could not look on the sweet heaven, A
Either at morn or eventide. B (13-16)
Because the last words in each line rhyme in an alternating pattern, these four lines are written in rhyme scheme that would be noted as ABAB.
Tennyson doesn't stick to ABAB the whole time, though. Here's the rest of that stanza:
After the flitting of the bats, C
When thickest dark did trance the sky, D
She drew her casement-curtain by, D
And glanced athwart the glooming flats. C
She only said, "The night is dreary, E
He cometh not," she said; F
She said, "I am aweary, aweary, E
I would that I were dead!" F (17-24)
Give this pattern of end rhymes, we can say that each stanza is written in an ABAB CDDC EFEF rhyme scheme. Don't worry, though. That pattern will not be on the test (at least, we hope it won't).
Something else is afoot in this poem, though, form-wise. Tennyson also uses meter to establish the rhythm of the poem.
Try reading the following aloud, paying attention to the way the syllables sound:
About a stone-cast from the wall
A sluice with blacken'd waters slept, (37-38).
It probably sounded something like this:
daDUM daDUM daDUM daDUM
daDUM daDUM daDUM daDUM
Each set of two syllables (each daDUM) is called a foot, and each foot is made up of an unstressed syllable followed by a stressed syllable. This kind of syllable pair is what's known as an iamb. And because each line has four iambs (four daDUMs), the line therefore written in iambic tetrameter (tetra- meaning four).
Most of the poem follows iambic tetrameter, but there are a few lines that do their own thing. For example:
I would that I were dead! (12)
daDUM daDUM daDUM
We still have daDUMS, or iambs, but there are only three feet in a line like this. Therefore, the line is written in, you guessed it: iambic trimeter.
So why the change when it comes to these lines? Why didn't Tennyson stick to his metrical guns and keep each line's meter consistent? One idea is that the break in the regular, established rhythm of the lines really drives home how down in the dumps poor Mariana is. One risk of having a super-regular meter and rhyme scheme is that the poem becomes sing-songy. Changing things up with some iambic trimeter, though, eliminates any chance of that.
We're instead left with an off-sounding line that really drives home how off our speaker is. (Check out "Speaker" for more on the depths of her bummed-out-ness.)