Iambic Tetrameter
"The Solitary Reaper" is a classic example of iambic tetrameter, which—as you can probably guess—is lot like iambic pentameter. Still, if you have no idea what the blazes we're on about, don't fret. We're just talking about the number of feet (rhythmic beats) that the poet uses in each line.
In our case of iambic tetrameter, we get four ("tetra-" means four) iambs per line instead of five ("penta-" means five) in iambic pentameter. And an iamb is just an unstressed syllable, followed by a stressed syllable (it makes a daDUM sound, like in "allow"). Take a look at line 1 as an example:
Behold her, single in the field.
The exact same pattern pretty much holds through the rest of the poem—pretty much. There are some notable exceptions, though. Just a few lines into the poem, we get:
Stop here, or gently pass! (4)
Here, we only get three iambs (this makes it an example of iambic trimeter). The same goes for lines 12 and 20. So, it looks like the fourth line of every stanza is off in this same way, until in the final stanza we get:
And o'er the sickle bending;— (28)
Don't let the extra beat at the end of the line (the –ing in "bending") throw you, though. The pattern of iambic trimeter is still technically in place. These little add-on beats at the ends of lines are not typically counted against the poem's prevailing meter.
In all, then, we have 28 lines of iambic tetrameter and 4 lines of iambic trimeter. Those 32 total lines are arranged into 4 stanzas that have 8 lines each (that's called an "octet" in the poetry biz). Anyway, each of those 4 stanzas (except the first and the last, which we'll get to in just a moment) has the following rhyme scheme: ABABCCDD, where the letters represent each line's end rhyme. The first four lines alternate (ABAB), and the last four are rhymed couplets.
The exceptions to this neat little pattern are lines 1 and 3 (according to the pattern, "field" and "herself" are supposed to rhyme but, yeah, they don't), and lines 25 and 27 ("sang" and "work"). Was Wordsworth unable to find words that rhymed? Did he just forget? It's possible, but unlikely. Wordsworth was a smart guy, and very meticulous about his rhyme and meter—meter-iculous, if you will (and we will). So, why throw these wrenches in the works?
Well, when you think about it, it actually makes good sense. The speaker is observing a scene that, we assume, was pretty standard fare in rural Scotland (fields, grass, birds, the works). The kicker is that solitary reaper. The highland girl is totally unique—she is "single," and not just "single" as in "alone," but "single" as in one-of-a-kind. She kind of sticks out, so much so that the speaker is momentarily stopped in his tracks (and even encourages others to stop also), just like you would be if you saw a tiger riding a unicycle down your block.
So, just as the solitary reaper makes the speaker stop and observe, those hiccups in both the meter (the lines of iambic trimeter instead of tetrameter) and rhyme make us readers stop too. The speaker's puzzlement and awe are kind of what the poem's rhyme and meter are supposed to capture. Those funky, out-of-place lines arrest our attention and stick out like little metrical sore thumbs. They seem out of place, unique—hey, just like the solitary reaper.