A speaker flings a famous line at us, declares it nonsense, and eight lines later is still stuck in the same place. That is the joke of Hopkins's miniature, and oddly it is also the argument. The line in question, "The child is father to the man", is best known from Wordsworth. Here it is treated as lunacy: "How can he be? The words are wild." The voice sounds like an exasperated schoolmaster who wants the world in proper order, with fathers first and children following.
The form does most of the work. This is a triolet: eight lines, two rhymes, and a refrain that keeps coming back. Line one returns as lines four and seven, and line two returns as line eight. The rhymes are *man*, *can* and *ran*, and then *wild* and *child*. The shape is a loop, and the paradox is a loop too. The child fathers the man, the beginning shapes the end, and in a triolet the opening line really does return to govern the close. The poem enacts the claim it says it cannot understand.
The speaker also loses on points. The sensible version, "The man is father to the child", appears once, wedged into line six. The "wild" version appears three times. The correction is outvoted by the structure. Small changes in the refrain add to the effect. When the quotation returns in line seven it gains an exclamation mark, and it is hard to say whether this is the speaker's disbelief or a quiet cheer for the paradox. Then line eight repeats the question "How can he be?" Now it comes straight after the "correct" version, so the question is no longer a request for a ruling. The tidy line has settled nothing.
Sound and diction are doing quiet damage to the speaker as well. "Wild" and "child" are the poem's only pair of rhyming words for the second sound, so the ear links childhood with wildness before the speaker has noticed. The complaint accidentally confirms the saying. "Suck any sense from that who can" is the most vivid line. The verb is physical and impatient, like squeezing juice from a stone, and to my ear it also has something babyish about it, which would suit a poem about children. The inverted "who can" turns the sentence into a dare. Almost every word is a single syllable, and only "father", "any" and "poet" run longer. That plainness leaves the paradox bare, with no ornament to hide behind.
The rhythm helps. The lines run in steady four-beat iambics, with the beat landing on "The CHILD is FA-ther TO the MAN", like a nursery rhyme. Hopkins is remembered for sprung rhythm and dense, crowded lines, so this bouncing regularity shows him in a lighter, more playful mood. The tune is the kind a child could chant, which suits the subject.
The best moment is line five: "No; what the poet did write ran". The speaker is confident, even a little smug, about what the poet "did write". But if the poet is Wordsworth, as the quotation suggests, then he wrote the paradox, and the speaker's "correction" is the one that is wrong. The speaker has rewritten the poet to fit common sense, which is the very mistake the poem is laughing at. The idea behind the saying is that the feelings and habits of childhood shape the adult we become, so in that sense the child does father the man. The poem never says this. It leaves the reader to supply the sense that the speaker cannot find.
That is why so small a poem matters. In eight short lines it shows form making an argument, with repetition standing in for explanation. It also teaches a lesson about reading. A line that sounds wild may sound that way because it is true in a way that ordinary logic misses. The speaker keeps asking "How can he be?" and the poem answers by simply going round again, with the beginning returning at the end.