The title tells us that the speaker's thinking of home, and the first line lets us know that that home, for him, is England. That "Oh" to begin adds a kind of emotional intensity to the rest of the poem. We know that this dude is going through some serious homesickness.
whoever wakes in England Sees, some morning, unaware, That the lowest boughs and the brushwood sheaf Round the elm-tree bole are in tiny leaf, While the chaffinch sings on the orchard bough In England—now! (3-8)
What's worse for the speaker than missing his home? It's the fact that other folks are back there, enjoying it. In the meantime, all he can do is imagine what they're experiencing in England, right down to the tiny leaves and birds. The level of detail here points to how intensely the speaker wants to be back.
And after April, when May follows, (9)
Here the speaker leaps forward in time. He's not only imagining what his home is like now, but what it will be like in a month. This move tells us just how connected he is to England, right down to the subtle shift in seasons that the space of a month might bring.
Hark, where my blossomed pear-tree in the hedge Leans to the field and scatters on the clover Blossoms and dewdrops—at the bent spray's edge— That's the wise thrush; he sings each song twice over, (11-14)
What's most telling about this passage is the word "my." Either the speaker is imagining an orchard in his own back yard at home, or he's so connected to this scene that he feels possessive of its details. We know feel that way about the pizza joint around the corner.
Far brighter than this gaudy melon-flower! (20)
Sometimes what brings home most into focus is the sheer difference you find in your surroundings. Based on this line, it's a safe bet to say that England has a lot of things going on in spring, but melon flowers sure aren't one of them. It's too cool, apparently. We didn't think it was possible to hate on a flower, but the speaker's description of the flower as "gaudy" tells us just how annoyed he is to be seeing it. He'd rather enjoy the more familiar details of his home.