It's almost as if Auden is toying with us at the start of this poem. See, the first couple of lines all have ten syllables in them. (Don't believe us? Count the syllables of lines 1-3. Don't worry. We'll be here when you get back.) That might lead some of you to think that he's about to set out writing in pentameter, which is comprised of five two-beat feet (or ten total beats).
Don't get too excited, though. Just when you think that Auden's settling into that oh-so-traditional of meters, line four derails everything completely. Line four goes on waaaaay longer than ten syllables. And after that, it's all up for grabs. It's almost as if "real life" – all that eating and opening windows and walking which the speaker describes – refuses to be contained by anything as stodgy or constraining as a formalized metrical pattern. That's what we mean when we day that the form is pretty free and easy. Life just seems to take over, and Auden's totally content to just let things roll.
The way that lines break throughout the poem helps to facilitate this sense of ease. (Believe us, it's a lot more carefully crafted than it might seem. Remember how hard it was to be effortlessly cool in middle school? Exactly.) Notice how most of the punctuation in the first few lines actually occurs in the middle of the lines? That means that lines tend to be enjambed – that is, they run on into each other, almost as if a speaker can't quite shut up at the proper time. Auden might just be thumbing his nose at the sorts of conventions which "old masters" of poetry would have recognized: rhyme, meter, and end-stopped lines. Why? Well, we're not totally sure. But chances are that he's trying to formally duplicate the sorts of entanglements and messiness which his poem tries to tackle.
We've said it before, but we'll say it again here: the deliberate references to "Old Masters" and museums and old paintings creates an interesting tension with the simple language and prosaic qualities of Auden's form. We've got two basic thoughts about this:
1. The disparity between the two is an intentional move, one intended to demonstrate how even simple subjects and language can be part of a great tradition.
2. The tension could be a way for Auden to demonstrate that there is no tension between great art and commonplace emotions. Even mythical characters get scared when they're falling. Especially when they're falling out of the sky. It's not necessary to come up with a spiffy new form to explain that!