Time let me hail and climb Golden in the heydays of his eyes (4-5)
In the beginning of the poem, the speaker personifies time as something with eyes. The speaker felt like he was under the watchful gaze of time. And because he says, "the heydays of his eyes" (5), we know that the speaker believes he's in his prime. Time's prime, that is, as if time was at his best when the speaker was young. Life couldn't have been better.
Time let me play and be Golden in the mercy of his means (13-14)
Time seems like an awesome guy here, right? The speaker says, "[t]ime let me play and be" as if time is giving him permission to play and just "be." The speaker doesn't have any worries and is just enjoying himself, and it's all thanks to time, who's a kind, benevolent, compassionate figure here. So what happens between now and the end of the poem?
In the first, spinning place […] (34)
Okay, what in the world is the "first, spinning place?" Well, the speaker doesn't say the word time, but we still think he's referring to time here. It's the beginning of a new day, and the speaker feels refreshed and like the day is brand new. You know that phrase, "every day is a new beginning?" Well, the speaker literally feels like he's waking to a day as new as the first day of time.
[…] that time allows In all his tuneful turning so few and such morning songs (42-43)
Oops, sounds like something's changed here. Time is still personified as a him, and this time the speaker is concerned with "his tuneful turning so few and such morning song." What's up with that, time? What was a merciful bounty has become something that "turns" and only has a "few" morning songs. The speaker is beginning to experience the passing of time and it's not like he thought it would be.
This is also the section of "Fern Hill" that might allude to the Pied Piper of Hamelin. The Pied Piper played a flute that led children away from their town. He was owed a debt and the town didn't pay, so he took the children as payment. In the same way, time is playing a tune that leads children away from youth. It lends a creepy note to a so far joyful poem.
[…] that time would take me Up to the swallow thronged loft by the shadow of my hand (46-47)
Throughout the whole poem, everything has been shining and light and golden, and time has been on the speaker's side. But not in this last stanza. Here time has taken him by the wrist and begun to lead him away. Suddenly, we don't see the speaker in light, but as shadow himself. And time's no longer the passive, watchful figure that he was at the beginning of the poem. Here, he's acting directly on the speaker.
I should hear him fly with the high fields And wake to the farm forever fled from the childless land. (50-51)
Time is folding up all the pastoral beauty the speaker loved and skipping town in the middle of the night. What's important here is that the speaker doesn't notice until it's too late. It's not that he watched as time packed up and drove away. Rather, he wakes and all that's left is the "childless land."
Time held me green and dying (53)
Time holding the speaker can be interpreted several ways. For example, time could have held him as if you might hold someone who is sick or "dying." But also, time could hold him like a baby, as in, cradled against his chest. At this point in the poem, we know that the speaker's view of time has changed from the beginning, so his relationship with time is complicated: part-protector, part-destroyer.