Nothing gold can stay. (Title, 8)
The speaker seems to think that the idea that "Nothing Gold Can Stay" is so important that he uses it for both the title and the last line of the poem. Ironically, the repetition and prominent placement of this line make it so that, even though nothing gold can stay, this line sticks in the reader's head. We guess Frost didn't think his truisms were gold.
Her hardest hue to hold. (2)
This line personifies nature to let us know about the transience of gold, which line 1 claims is nature's first green. It makes us think of nature as a teenage girl squeezing lemon juice or blond dye in her hair to try to make it golden-blond. But, alas, the golden colors just weren't meant to last for long, and no matter how hard she tries, the gold will always fade.
But only so an hour. (4)
Again, the poem establishes the transience of spring, of golden trees and their blooms. "Hour" is figurative here; after all, trees can bloom for weeks. Perhaps when something is so beautiful that you could stare at it forever, a week might as well be as short as an hour.