And the May month flaps its glad green leaves like wings, Delicate-filmed as new-spun silk […] (2)
The month of May is described as a bird or winged insect that "flaps" its "leaves like wings." The leaf/wings are "delicate-filmed," like a baby bird's or a butterfly's. This image connects to the word "tremulous" the poet uses to describe his own fragile, uncertain life in line 1. "New-spun silk" could also suggest butterflies coming out of silken cocoons, so there's a hinted allusion to metamorphosis or transformation in this line.
If it be in the dusk when, like an eyelid's soundless blink, The dewfall-hawk comes crossing the shades to alight Upon the wind-warped upland thorn, […] (5-7)
The "dewfall-hawk" is a hawk that comes out at "dewfall," or dusk. It glides silently ("like an eyelid's soundless blink") across the shadows to land on a thorny shrub. Notice all the compound words here, like "dewfall-hawk," and "wind-warped."
[…]mothy and warm (9)
The darkness is described as "mothy and warm" in this stanza – "warm" we understand. It's a warm summer night. But describing the darkness (or "nocturnal blackness") as "mothy" seems a bit more unexpected. Hardy sure was fond of those unexpected images. So what does "mothy" do? Well, it brings to mind those warm summer nights when the moths flutter around the lights. The word could also connect back to the butterfly imagery of the first stanza ("delicate-filmed as new-spun silk").
When the hedgehog travels furtively over the lawn (10)
Here's another unexpected, unusual nature image. The "hedgehog" is moving "furtively," or cautiously, across the grass. It's nighttime – why is it being so cautious? It could connect back to the "dewfall-hawk" of the previous stanza.
Watching the full-starred heavens that winter sees (14)
Wait a minute – "winter" can't "see" anything – it's a season, not a person. It's being personified (go to the "Symbols, Imagery, Wordplay" section for more on that). The neighbors who are "watching" the stars are being subtly, or implicitly, compared to "winter," which also "sees" the stars. So maybe people and nature aren't as separate as they might seem?
"He was one who had an eye for such mysteries (16)
The speaker of the poem is also connected to the neighbors and to the "winter" through the same idea of watching the stars. He's a person who "had an eye" for "mysteries" like stargazing.