I cannot rub the strangeness from my sight I got from looking through a pane of glass I skimmed this morning from the drinking trough And held against the world of hoary grass. It melted, and I let it fall and break.
These are the most confusing lines of the poem, because they refer to a phenomenon that few people have to deal with nowadays.
Basically, the speaker remembers how, earlier that morning, he went outside to get drinking water from his trough. That's right: his water does not come from a tap.
The night was cold enough to freeze the top layer of the trough into a sheet of ice, and the speaker picked up ("skimmed") this sheet to get at the sweet, sweet water beneath. Then he held up the ice sheet and looked out at the world through it. The speaker saw the frost-covered ("hoary") grass, distorted by the mirror ("glass") of the ice. It's like looking at a fun-house mirror, except it's not really a mirror, because he is looking through the ice.
Then the ice started to melt, probably from the warmth of his hand, and he let it "fall and break" against the ground. End of story.
What's the point of all this? According to the speaker, it made his vision seem "strange," as if he was looking through a distorted lens, and he hasn't been able to get rid of this sense of strangeness all day. He has tried to "rub" it from his sight, like you might rub the sleep out of your eyes in the morning, but it hasn't worked. The whole world continues to look odd.
But I was well Upon my way to sleep before it fell, And I could tell What form my dreaming was about to take.
Whoa, time shift. The speaker says he was "upon [his] way to sleep" before the ice dropped, which doesn't make any sense unless you consider that he is already starting to dream about the memory.
In real life, he dropped the ice, but in his thoughts before sleep, he cut away from the image before the ice could fall.
We're now thinking that it is more likely that the speaker is in bed after a long, hard day than that he is still up on the ladder. Which is good, because it would be kind of dangerous to fall asleep on a ladder.
He guesses what he will dream about when he falls completely asleep.