Little we see in Nature that is ours; (3)
The speaker suggests that humanity no longer feels any connection with nature; in a sense, nature no longer belongs to us, or we don't relate to it. This disjunction or separation implies that nature is dead, in that it has become something of an alien or foreign presence.
We have given our hearts away, a sordid boon! (4)
If we were to give our hearts away literally we'd be dead; the fact that we are no longer moved by nature is a kind of death. According to the speaker, in order to be truly alive we have to be in touch with nature, otherwise we're just empty bodies with no hearts. We're "dead" to nature, which is just like being six feet under.
It moves us not. (9)
The word "move" suggests that people are at a stand-still, almost as if they were corpses. While "move" is here a metaphor, one can't help importing the more literal meaning that lurks beneath, especially considering the elegiac tone that permeates the poem.