How we cite our quotes: The poem is 159 lines long, so we just cite by line number.
Quote #1
Once again
Do I behold these steep and lofty cliffs,
That on a wild secluded scene impress
Thoughts of more deep seclusion; (4-7)
The speaker is back on the banks of the Wye, and the view is staggeringly beautiful. The "steep and lofty cliffs," especially, inspire him with awe. Perhaps it's because of their height and dramatic steepness that he is particularly drawn to the cliffs.
Quote #2
Nor less, I trust,
To them I may have owed another gift,
Of aspect more sublime; that blessed mood,
In which the burthen of the mystery,
In which the heavy and the weary weight
Of all this unintelligible world,
Is lightened: (35-40)
The speaker's memory of the "beauteous forms" (22) not only cheers him up when he's feeling bummed, but also makes him feel as though the weight of the world "is lightened." He calls the sensation a "blessed mood." This is the first hint of the almost religious devotion that the "presence" in Nature will inspire in him.
Quote #3
that serene and blessed mood,
In which the affections gently lead us on,—
Until, the breath of this corporeal frame
And even the motion of our human blood
Almost suspended, (41-45)
When the speaker remembers the "beauteous forms" (22), he's not so distracted by the immediate sensations of the "steep and lofty cliffs." He's able to reflect more. And that thoughtful reflection leads to the almost meditative trance that he describes here. His breath slows down and even his heart rate seems to drop.