This line shows us what happens after the early leaf is no longer figuratively a flower—it becomes a true leaf. But the speaker doesn't say "becomes," he says "subsides." This means that the first leaf sank down, or settled, to become another leaf.
The use of the word "subsides" implies that the speaker thinks that the first leaf—the flower of sorts—was better than the actual leaf. The first leaf had to stoop down, or lower itself, to become the second one.
Going with our two interpretations, this means that either spring blooms and flowers are more beautiful than the full leaves of summer, or that leaves in the early morning are much prettier than leaves at, say, midday.
So Eden sank to grief,
The "so" at the beginning of the line means "just as," which means that in line, the speaker is comparing mankind's fall from the Garden of Eden to the change from gold to green he described in the first half of the poem.
Just like the flowers subsided, or were downgraded to become leaves, Eden also sinks. This refers to the biblical fall of Adam and Eve from the Garden of Eden. When Eve ate forbidden fruit, she and Adam, and all their descendents, were forever punished by being banished from Eden and subject to all of the grief that we humans know today.
Both these events may seem like a big bummer, but keep in mind that the fall of Adam and Eve is often viewed as a fortunate fall, because even though it was tragic that humans sinned, their sin set the stage for all of history to be set in motion. In other words, if Adam and Eve hadn't gotten the munchies, Shmoop wouldn't be here explaining this poem to you. And that would be a travesty.
So, the poem suggests, just as it was tragic that the first biblical humans were expelled from the Garden of Eden, it's tragic that the early golden flowers of spring and dawn grow into mere leaves. But it's possible that there's an upside to all this sorrow, too. Or at the very least, it's the way it goes, and we can't change it.
So dawn goes down to day.
This line is yet another comparison to the colors of spring turning from gold to green, and the fall of Adam and Eve, but this time, dawn is going down to day.
Think of refreshing, golden hues of sunrise turning into the scathing heat of a summer day.
The use of the word "down" in this line makes us think… isn't the sun supposed to rise in the morning? Wouldn't dawn rise up to day? Well, yes, literally, the sun does come up. But the speaker prefers dawn. Day is at a lower level because, possibly, it's lost the freshness and color of the dawn.
We think again, though, of the fortunate fall. It's a good thing, Shmoopers, that dawn turns into day, even if the daytime horizon is significantly less spectacular than a sunrise. Sunshine is wonderful—it helps the plants grow and feels nice and warm after a chilly morning.
Nothing gold can stay.
This last line connects us back to the title and reminds us of the only other mention of gold in this poem, which is in the first line. The line neatly wraps up all the metaphors: early spring leaves and flowers, the Garden of Eden, and dawn are all gold, and none of them can stay for very long in this world.
Gold—after its brief appearance as a beautiful color, or a feeling of contentedness, or whatever is fresh and new and wonderful—has to hit the road, leaving us with green leaves, people who know sin and guilt, and the bright, boring light of day.
But take comfort. Even though we'll eventually lose the glaze of gold, now that we know it's not sticking around, we can appreciate it more. Also, we can go on with our daily lives. Leaves can soak up sun and rain to feed trees, we can enjoy all the wonderful complexities of knowledge, and the full light of sun can blaze on.