Nothing happens in "It Rains", and that is exactly the point. A speaker stands in an overgrown orchard, watches the wet, and lets his thoughts drift toward a happiness he can only imagine or remember. The central idea is that joy and loss can't be pulled apart. He is "nearly as happy as possible" and "sad, too" in the same breath, and the poem never tries to settle which feeling wins.
The first stanza builds its world mostly out of absence. "Nothing stirs," the forest of parsley is "untrodden," and the raindrops are ones "there is none to break." The negatives pile up until the orchard feels like a place left entirely to itself. Edward Thomas also plays with scale. A tangle of parsley becomes a "dense / Forest," and drops on grass become "great diamonds." Small, humble things are given the grandeur of jungle and jewellery, and the speaker is almost the only person to notice. The line "The great diamonds" ends at the line break, so the word hangs there for a moment, as a drop does before it falls.
The second stanza turns inward. "To search the wilderness in vain though well" is a knotty, tentative phrase. The search is fruitless, yet somehow it is done "well", and the poem doesn't say what is being searched for. What follows is a picture of "two walking, kissing there, / Drenched, yet forgetting the kisses of the rain." The rain is personified as a lover, and the imagined couple are so absorbed in each other that they ignore it. The speaker, by contrast, is attending to the rain fully because he is alone. The grammar then does something clever. "Never, never again" ends the stanza, and the sentence spills over the stanza break into "Unless alone, so happy shall I walk." The repeated "never" sounds like grief, but the next line qualifies it at once. Happiness is still possible, only in solitude.
In the last stanza the light fails. The syntax tangles here, and the tangle seems deliberate. "Twilight has fined to naught" the flower's "fine stalk", and the repeated "fine/fined" makes the stalk seem to thin out into nothing as the dusk deepens. Then the flower "figures, suspended still and ghostly white." "Figures" can mean that it appears, and it can also mean that it stands as a symbol. Either way, the flower becomes a ghost, and the final line says what it is a ghost of: "The past hovering as it revisits the light." The word "suspended" echoes the raindrops of the first stanza. A drop on a grass blade and a pale flower in the dusk are both held for a moment between presence and disappearance, which is how the poem treats memory too.
The form supports this mood. Each stanza has five lines, and each loosely follows a pattern of rhymed pairs around an unrhymed middle line: "fence/dense" and "break/shake" around "diamonds", "walk/stalk" and "white/light" around "flower". The middle line of the second stanza, "there", is left without a partner in the same way. These orphaned lines are where the poem's images and turns of thought gather, as if they were left standing alone like the speaker. The rhyme in the second stanza is looser, with "possible" and "well" sharing only a final sound, and that makes the stanza sound less settled, which suits its uncertainty. The rhythm sits near the beat of ordinary speech, close to iambic but constantly loosened by enjambment, so the sentences wander across line endings as a walker wanders through grass. There is no grand vocabulary, and the emotion comes through understatement. "Nearly as happy as possible" is a modest, almost comic phrase, but it holds a great deal of restraint.
That restraint is much of what makes the poem significant. Edward Thomas, who was killed in the First World War in 1917, never mentions war here, and the poem should be read on its own terms. A reader may still feel that the "never, never again" gives the quiet scene a deeper shadow. What the poem offers is an honest account of mixed feeling. It doesn't console or explain. It shows a man in the rain finding something close to happiness in the same place that makes him feel its loss.