Three lines, and the speaker admits defeat in the first two. That is the quiet joke of Shelley's "Fragment": a poem about being unable to describe something, which manages in a few words to make us feel what it cannot say. The subtitle tells us the lines are "adapted from the Vita Nuova of Dante," so we are hearing an older idea in a new voice: the lover who finds that language collapses in front of the beloved.
The situation could hardly be simpler. A woman called Mary gives a small smile, and the speaker is so overwhelmed that he can neither put the effect into words nor recall it. The central idea is that the strongest experiences slip out from under language. The failure is also double. He cannot "tell," and he cannot "call to mind" either. The smile defeats both speech and memory, which is a bigger claim than the usual lover's "words fail me."
The poem's construction is calm, which makes it interesting. All three lines are unrhymed iambic pentameter, ten syllables each, with the beat falling clearly: "What MAry IS when SHE a LITtle SMILES." The speaker's mind may be reeling, but the metre is not. That steadiness makes the astonishment feel like a reverent hush rather than a gasp. The lack of rhyme matters too. "Smiles," "mind" and "rare" never chime, so the poem has no closing click and seems to hang open. For a piece called a fragment, that feels right.
The syntax does some quiet work as well. The first sentence begins with its object, "What Mary is when she a little smiles," so the thing that cannot be described is placed before us, and we must wait until the second line for the confession, "I cannot even tell." The word order inside the clause, "she a little smiles," is slightly old-fashioned and courtly. It also puts "a little" right beside the smile, so we notice how small the cause is. The poem runs on this mismatch. The gesture is the faintest one a face can make, and the speaker's response is to reach for the word "miracle."
That word stands out because almost everything around it is plain. Apart from "Mary," "little," "cannot," "even" and "miracle" itself, nearly every word is a single syllable. "Miracle" is the only three-syllable word in the poem, and it sits in the middle of the last line like the one large object in a bare room. The plain diction makes the speaker sound honest and without ornament. The small word "even" adds to this. "I cannot even tell" suggests a lowering of ambition: he cannot describe the smile, and he cannot manage even the lesser task of remembering it.
The last line does the most with the least: "It is a miracle so new, so rare." The doubled "so," split by a comma, gives two beats of wonder that sound almost like a stammer. "New" also explains the failure, since language is built from past experience and this has no precedent. "Rare" adds the sense of value, something seldom seen and not to be wasted. And "miracle" gives the moment a religious weight. The smile seems to break the ordinary rules, which fits the reverent atmosphere of Dante's book about his love for Beatrice.
The name "Mary" is the one concrete anchor in the poem. Since the Vita Nuova is Dante's book about Beatrice, the name here moves the feeling into a new, named setting. Readers who know that Shelley's wife was called Mary may hear a private note, but the poem never says so, and nothing in the text confirms it. The name works either way, because it makes the abstract wonder belong to a particular person.
The poem's significance lies in how neatly it enacts its own subject. A longer description of the smile would almost certainly diminish it. By refusing to describe, and by saying only that the smile is new and rare, the poem leaves us to imagine it, and what we imagine is probably more vivid than anything the poet could list. It also shows a poem passing between centuries and languages, with an old idea made fresh in plain English. In three lines the poem is complete as an expression of speechlessness, even though it calls itself a fragment.