Ballad
Wordsworth wrote "We Are Seven" in the form of the ballad, which is one of the oldest forms of poetry. Ballads are often stories told in verse, and "We Are Seven" fits the ballad bill perfectly.
The basic ballad has three components: it's made out of quatrains, it has an ABAB rhyme scheme, and it has a regular meter in which iambic tetrameter lines alternate with iambic trimeter lines.
Now, don't go getting all nervous about these scary poetry terms. We're here to break it down for you, Shmoopers. First up, a quatrain is a just a 4-line stanza. Easy as pie so far, right?
Secondly, an ABAB rhyme scheme means that the first and third lines of each stanza rhyme with each other, as do the second and fourth lines. For example, in these lines from "We Are Seven"
The first that died was sister Jane;
In bed she moaning lay,
Till God released her of her pain;
And then she went away.
"Jane" rhymes with "pain," and "lay" rhymes with "lay." Once again: easy as pie.
Third: meter. Ballads almost always alternate iambic tetrameter lines with lines of iambic trimeter. An iambic tetrameter line has four beats ("tetra-" means four), while an iambic trimeter line has three ("tetra" and "tri-" means three). Those beats are each made of iambs, or two-syllable units that are unstressed on the first syllable, stressed on the second.
Put all that together and we get this pattern:
daDUM daDUM daDUM daDUM
daDUM daDUM daDUM
Or, in the case of the stanza above, we get:
The first that died was sister
Jane
In bed she moaning lay
You should hear that pattern in the beats of the lines. Now, "We Are Seven" is written in a pretty regular metric form. Wordsworth doesn't change things up too often; he's clearly committed to the ballad. There are two exceptions, though: the meter of the first stanza is kind of whack. It's like the speaker is clearing his voice and not quite into the poem yet:
--A simple Child,
That lightly draws its breath,
And feels its life in every limb,
What should it know of death?
You just don't get the regular ballad meter there. In a way, that indicates the kind of warm-up moves—not quite in the flow—that we mentioned the speaker doing earlier. The last stanza is even more of a doozy. Wordsworth adds an extra line to the quatrain and plays around with its rhyme scheme, which then becomes ABCCB. This formal disruption seems appropriate in its own way, as well. The first line—"But they are dead; those two are dead!"—is left hanging without a rhyming partner, but perhaps because the dead themselves have no equivalent (rhyming) partners. As well, the extension of the final stanza really sets up, and then drives home, the little girl's final say on the matter.
Form-wise, then, the ballad story of the poem is bookended by odd ball stanzas—a warm-up to start with and an emphatic extension to wrap it all up.