"Alone" lives somewhere in the no man's land between formal regularity and an absolute free-for-all. There are some absolutes: every other stanza, for example, is exactly the same. (We're talking about stanzas 2, 4, and 6 here).
The first, third, and fifth stanzas are another story. They all have nine lines – unless you count the first stanza, which has ten. (We could argue that the first two lines of the poem are actually one split line, but that's another story.) And those lines tend to have six or seven syllables – unless, of course, you're talking about the 7th or 8th line. Those have four syllables each.
Confused yet? We don't blame you. Here's what we do know: there's not a metrical or formal regularity to this poem. There is, however, a sort of formal logic to the way that the poem's narrative unfolds.
Think of it as a camp song: the camp counselor lays out a little bit of a story (six lines of it, to be precise) and then sings a verse that's easy to remember (in terms of our poem, this is always the last three lines of the stanza). And then the campers sing the verse back to the counselor. (That's the second, fourth, and sixth stanzas.) It's easy to remember because, well, if you're in the chorus, you don't have all that much to remember.
Come to think about it, these sorts of call-and-response songs have been popular for centuries. They were a big part of church traditions back when it wasn't common for everyone to have hymnals. They're the format most military marching songs tend to take: the C.O. shouts something out, and the squad shouts back a reply. They're the core of most oral traditions – when you aren't able to write everything down, it's good to have a refrain as a sort of memory marker.