The speaker opens with the question that the entire poem revolves around: "Who has seen the wind?" We know that this question (and first line) is also the poem's title, which is a pretty conventional way to start a poem.
Remember, we're dealing with the Victorian era here, when poets weren't looking to be all experimental and modern. They followed the rules, in other words, and used popular meters and forms quite often. Check out "Form and Meter" for more.
So who has seen the wind? No one. The speaker is also assuming in line 2 that there's no way we would have ever seen the wind either. She's kind of calling our bluff, just in case we felt like being cheeky.
We notice that the speaker is also using first-person point of view in line 2. So she's speaking to us from a personal perspective, which makes us feel like she's talking directly to us.
In other words, the speaker is kind of telling us what's what, similar to the way a parent might explain things to their kids.
Notice we have a sort of clear meter in both lines: a headless iambic trimeter, which is just fancy (gruesome) talk for an unstressed/stressed syllabic pattern that has a total of three stressed syllables in each line. It's "headless" because it's missing that initial unstressed syllable in the beginning of the line. So it goes a little somethin' like this: DAdumDAdumDA.
Why the fancy meter? We talk more about that in "Form and Meter," but for now we can assume that such a catchy sound pattern helps make nursery rhymes like this stick. It's easy to predict the sound (and sometimes the words) that will come next.
So it can also be a useful learning tool for little kids.
But when the leaves hang trembling, The wind is passing through.
In line 3 we have a bit of personification. That's another literary device that's always appreciated by the kiddos. The figurative language we see here describes the moving leaves as "trembling," which is something people usually do rather than leaves.
So suddenly we're comparing a tree's leaves to a trembling person, which adds a fun layer to our imagination. And why are the leaves trembling? Because the wind is "passing through" in its mysterious and invisible way.
Although we can't "see" the wind, we know it's there since the leaves are moving around. The wind is invisible in the poem just like it is in real life, so the speaker isn't looking to make the wind appear as something different from what it is. She's keeping things relatively real.
Notice the speaker's simplicity in the way she describes things. Aside from the hint of personification, she sounds pretty straightforward. No fancy lingo here. And her simple descriptions mimic the way nature works, without any elaborate show or purpose. The wind is "passing through," in a rather effortless way.
We have a different kind of meter in these lines too: line 3 is written in iambic tetrameter (daDUM) with a total of four stressed syllables. Line 4 is in iambic trimeter (daDUM again) with a total of three stressed syllables.
Catchy? Check. Predictable? Check. Again, the meter's really sticking this poem in our minds.
And finally, notice that we have a perfect end rhyme between the "you" in line 2 and the "through" here in line 4. Now we know for sure that we're dealing with a neat little nursery rhyme.