Shelley's fragment opens with a puzzle: how can hope be a form of despair? The lines describe a mind in which every feeling that should help has turned against it. Hope has become "the sick despair of good", fear is no longer a suspicion but "the certainty of ill", and doubt feeds Expectation only on food that is "turned while she tastes to poison". The central idea is emotional paralysis. When the "will / Is powerless", even the feelings that normally push us forward become symptoms of the trouble.
The most striking thing about the construction is its syntax. Three lines begin with "Such", a repetition that sets up a sentence of the form "Such hope… such fear… such doubt… is…". The expected main verb never arrives. Each "Such… as is" clause tells us what kind of hope, fear or doubt is meant, but not what happens to it. Then the fourth line brings "when the will / Is powerless, and the spirit…", and the text breaks off in mid-thought. We cannot tell how Shelley meant to finish, but the fragment as we have it enacts its own subject. The sentence hangs, waiting for a resolution it never gets, just as the speaker's mind waits.
The metre helps this feeling of suspension. The lines are iambic pentameter, and they are very regular: "Such HOPE, as IS the SICK deSPAIR of GOOD" ticks along evenly, like a steady pulse or a fixed argument. The rhymes alternate, *good* with *food* and *ill* with *will*, which sets up the pattern of a formal stanza. The poem looks orderly, and that order sits oddly against the disorder it describes. The rhyme pairs are also meaningful. "Good" rhymes with "food", and that food is promptly poisoned. "Ill" rhymes with "will", so the sound of evil or misfortune is bound to the sound of the faculty that ought to resist it, and the will loses. The third line also runs on into the fourth with no pause, so the poison seems to spill across the line break, and the mind has no time to recover.
The imagery is compact and physical. Hope is "sick", which treats an emotion as an illness. Expectation is personified as a woman, "pale" and presumably underfed, who is offered "food" that is really poison. Shelley compresses time into the single word "while": the food turns "while she tastes", so there is no moment of satisfaction. Expectation is cheated at the very instant of tasting. The capital letters on "Expectation" make her a figure of allegory, a kind of ghost in the mind, rather than a mood the speaker simply has.
The phrases also open into more than one meaning. "Despair of good" can be read as the despair we feel about goodness, meaning the loss of faith that anything good can happen. It could also be read as the despair that good people feel. Either way, hope is no longer the opposite of despair here but a version of it, a feverish clinging that is itself a sign of illness. Similarly, "the certainty of ill" turns fear, which normally depends on not knowing, into something grimly settled. The language works by twisting familiar pairs until they lose their usual meaning. Hope sits beside despair, fear beside certainty, and doubt feeds on expectation.
What makes the fragment worth reading is how much it does in so little space. Shelley is widely known as a poet of idealism, so a passage in which hope is poisoned is a useful counterweight, a glimpse of the darker psychology that such idealism has to face. The abstract nouns (hope, fear, doubt, will, spirit) could have made the lines cold, but the sickness, the pallor and the poison keep them physical. The broken ending then adds to the effect. The fragment stops at "the spirit…", on the word that names what is at stake, and leaves the reader with the same incompleteness the lines describe.