Though Williams claimed in an interview with John Gerber (source), that "This Is Just To Say" was metrically regular, meaning that it has a regular rhythm, he must have meant this visually, and not in the traditional sense. There is no regular beat in this poem, though it is fun to read aloud and rolls smoothly off the tongue. That means we're working with free verse, dear Shmoopers.
Yet the attention to line breaks and form is very deliberate in this poem. This isn't some free for all, scribbled on the back of a grocery list. It's a poem. And poems are often all about how they appear on the page.
"This Is Just To Say" is divided into three four-line stanzas, and the lines are all about the same length—very short. There's no punctuation in the poem, which would seem to mean that it's just one sentence—one of those big ol' run-ons your English teacher might hate. Notice, though, that the word "Forgive" is capitalized in the third stanza, which may mean we're dealing with a new sentence.
The most distinctive part of this poem's form is its short short short lines. We mean, they're short on short, which can make things a bit awkward if you take them all as pauses. So if you're reading this poem aloud (which is always a good way to help understand a poem) keep in mind that a line break doesn't necessarily mean that you should go all silent on us. If there's no comma or period, just keep right on reading with no break.
Still, you'll notice as you read, especially the first few times, that even if there's no punctuation, you have to pause at the end of the line at least a little to let your eye skip down to the next line to know what's coming.
All of these little pauses make each little line seem more significant. The speaker seems to be having trouble writing this little apology. He feels guilty for eating these plums, and wants to write about it exactly. Think of the pauses as more mental or emotional than audible.
Finally, this is just to say, the line breaks also help this poem have a somewhat humorous effect. It surprises us with each new line. Seriously, dude? You're feeling bad about chowing down on some stone fruit?