Stanza 3 Summary

Get out the microscope, because we’re going through this poem line-by-line.

so world is a leaf so a tree is a bough

This line, repeating the word "so," jumps off from line 18. It seems that this line is the logical result of that previous line: "one's everyanything," therefore the world is a leaf, and a tree is a bough.
This makes us think of the world as rather changeable and magical. One thing can be everything, so if a leaf is one thing, then the world, representing everything, is a leaf. In the same sense, a whole big tree can be just one bough, or branch, and vice versa. Wow.
We would say that this line has two metaphors, in which the speaker would be comparing the world to a leaf and the tree to a bough, but we suspect that, in the world of this poem, this metaphor is actually literal.
After all, everything impossible is happening: every one thing could be any other thing, or everything all at once. This might seem crazy, but just remember – the rules of reality don't apply here. Everyanything goes.

(and birds sing sweeter
than books
tell how)

This parenthesis remind you of anything? How about lines 11-13?
Yep, just as "buds know better / than books / don't grow," these birds sing sweeter than books tell how.
The comparison, here, is between a bird's song and a book's explanation. Sure, a book could try to explain what a bird's song sounds like, but the description will never be quite as sweet as the actual sound. Shmoop thinks, though, that if you can't actually hear that song, then reading about it is the next best thing.
These are perhaps the most quoted lines of the poem, and, thankfully, they're actually rather easy to make sense of. Basically, the speaker is arguing that nature is a lot more brilliant than books could ever capture.

so here is away and so your is a my

This line echoes the structure of the first line of the stanza grammatically, but it doesn't mean the same thing. While the first line of the stanza says that a larger thing (like the world or a tree) is a smaller thing (like a leaf or a bough), this line is comparing opposites.
"Here," it says, is "away," and "your," is "a my." The extra "a" in front of "my" seems a little awkward, but the trick here is to read the line aloud – and then you'll hear the echo from "away" that plays into "a my."
This line hints a little more at the meaning of this whole "one" thing. What's far is near, what's yours is mine. In this poem at least, it seems that this big old crazy world all boils down to oneness. Everything is everything, or so Lauryn Hill would say.

(with a down
up
around again fly)

These lines seem to be describing the way that "here is away" and "your is a my." However, this line describes flying, which brings us right back to lines 20-22, where we're told that birds can sing sweeter than books can describe.
So now birds (or something else, maybe?) are flying down, up, and around in this world in which opposites are melding and large is becoming small. This is a happy, joyous existence, freed from the constraints of books and from the rules of space and time.
What's so fun about these lines in particular is that we might read them as an invitation or command. The speaker wants us to fly up, down, and around as we read this poem. We're totally down for some flight.

forever was never till now

The concept of "forever" seemed impossible, far away. It seemed as though it would never actually be true, that is, until now, at this moment, in this poem. Awesome, right?
Of course there are a couple different kinds of forever we could think about, but judging by the context of the poem, we think they are probably all happy.
Our speaker isn't thinking about "forever" as being condemned to live his afterlife in hell, or something like that. Instead, he could be envisioning eternal life in heaven, eternal time on Earth, or an eternal emotion, like true love. Remember, anything's possible.
But what is it about this very moment that makes forever possible? What has changed? Keep reading…