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.
more like a man Flying from something that he dreads, than one Who sought the thing he loved. (70-72)
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.
For nature then (The coarser pleasures of my boyish days, And their glad animal movements all gone by) To me was all in all. (72-75)
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.
For I have learned To look on nature, not as in the hour Of thoughtless youth, but hearing oftentimes The still, sad music of humanity, Nor harsh nor grating, though of ample power To chasten and subdue. (88-93)
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.
A motion and a spirit, that impels All thinking things, all objects of all thought, And rolls through all things. (100-102)
The speaker describes the "presence" that he has learned to detect in nature. It "impels," or animates "all thinking things," and connects everything.
Knowing that Nature never did betray The heart that loved her (122-123)
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.