Lines Composed a Few Miles above Tintern Abbey, on Revisiting the Banks of the Wye during a Tour, July 13, 1798 Man and the Natural World

The "forms" the speaker is describing are the shapes of the cliffs, trees, cottages, et cetera that he describes in the first stanza. "Forms" seems very non-specific, though. Why doesn't he say "landscape"? Considering the personal relationship he develops with this particular scene, it might seem odd that he uses such an impersonal word to describe it.

The speaker describes his younger self, the boyish William, running like crazy around the mountains of Wales. Maybe the younger William did have a chip on his shoulder ("something he dreads") from which he was trying to run away. Or maybe he was just running for the sake of running, with no purpose at all, because he hadn't yet learned to appreciate nature intellectually.

The "coarser pleasures" and the "glad animal movements" describes the younger William's crazy running around from the more mature point of view of the speaker. Yes, he says, nature was everything to him back then, but his "pleasures" were "coarser," or less sophisticated, than they are now.

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.

The speaker describes the "presence" that he has learned to detect in nature. It "impels," or animates "all thinking things," and connects everything.

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.