| Quote #4
For I have learned
Now the speaker has matured enough from his "thoughtless youth" to see beyond the immediate, physical pleasure in a beautiful scene. He's able to see something wider and more universal ("the still, sad music of humanity"), which helps to "chasten and subdue," or calm him down.
| Quote #5
A motion and a spirit, that impels
The speaker describes the "presence" that he has learned to detect in nature. It "impels," or animates "all thinking things," and connects everything.
| Quote #6
Knowing that Nature never did betray
The speaker has learned to rely on Nature (now with a capital "N"!) with almost religious devotion. His enjoyment of nature has turned into a kind of pantheism, or natural religion: he's learned to see a divine spirit in everything in nature, and he prays to it.