Petrarchan or Italian Sonnet, Sprung Rhythm
"Spring" follows the form of an Italian Sonnet. It has fourteen lines and uses the rhyme scheme ABBAABBA CDCDCD. Generally, sonnets are written in a single stanza, but it's not too uncommon for an Italian sonnet to break up the octet (the first eight lines) and sestet (the last six lines) into separate stanzas. In this poem, Hopkins goes even further, by also splitting that sestet in two.
We're guessing that Hopkins uses these stanza breaks to emphasize the changes in mental direction that his poem takes. The whole octet seems to follow from that first line: describing the beauty of spring. Then – with a stanza break – Hopkins switches gears with the question he asks at the beginning of the sestet. The third time, the shift happens mid-line, but the stanza break is right on its heels, and comes in time to prepare us for the sudden address to Christ. It's nice of Hopkins to give us these little visual cues, right?
The lines throughout the poem hover between nine and thirteen syllables each, which stays close to the typical ten-syllable lines of most sonnets. But Hopkins's big innovation is in his rhythm. He uses here, and throughout his poetry, something called sprung rhythm. In sprung rhythm, the stressed and unstressed syllables are arranged into little clusters, to create a sudden bursts or breaks in the speed or power of a line. So instead of composing all his lines in the ordinary iambic rhythm: da DUM da DUM da-DUM da DUM, Hopkins might go da DUM DUM DUM DUM da DUM DUM DUM. That's the rhythm in line 9:
What is all this juice and all this joy?
By clustering those stressed syllables together, it packs the line with a sense of emotional intensity and makes it harder to dismiss the question as vague or rhetorical.