"Methought" is the first word of Milton's sonnet, and it tells us none of this is real. It is the language of dreams, and the poem records one. The speaker sees his dead wife brought back to him, veiled and dressed in white, and loses her the moment she leans toward him. The central idea is that a consoling dream can be crueller than no dream at all. It gives him the sight of her that waking life denies, then takes it away and leaves the loss fresh.
The first eight lines build the return out of comparisons. The first is Alcestis, the Greek wife whom "Joves great Son" (Hercules) rescued "from death by force" and gave back to her "glad Husband." The second is the "old Law" of purification, the biblical rules for a woman "washt from spot of child-bed taint." Pagan myth supplies the miracle and scripture supplies the purity. The myth also works against the speaker. Hercules wins his gift by strength, but a dreamer has no strength, only hope. The sonnet reaches for a story with a happy ending and is then denied one.
The syntax enacts the longing. The sentence beginning "Methought I saw" runs through the whole octave, piling up comparisons, and its main verb, "Came," does not arrive until line 9. We wait through eight lines for her to appear, and the sentence spills across the usual break between octave and sestet as though the speaker cannot stop. Inside it sits the poem's one forward-looking line, where he trusts "once more" to have "Full sight of her in Heaven without restraint." The word "restraint" quietly sets up the ending, since he will be restrained in the dream, unable to complete an embrace.
The sound of the poem matters too. The octave follows the Italian pattern, abba abba, so the same sounds return four times. Its a-rhymes, "saint," "faint," "taint," "restraint," share a hushed, fragile sound. "Saint" and "faint" also sit at opposite ends of the first quatrain, holiness set beside frailty, and her return is described as "pale and faint." The sestet rhymes cdcdcd, and its d-sounds are "sight," "delight," and "night." That sequence is the poem's whole movement in three words.
The sestet describes her, and the description is a paradox: "Her face was vail'd, yet to my fancied sight, / Love, sweetness, goodness, in her person shin'd." The face is hidden, yet it shines. Milton gives no hair, eyes or smile, only three abstract qualities that "shin'd / So clear, as in no face with more delight." We never learn what she looked like, because for the speaker she is made of virtues rather than features. "Fancied sight" also admits that this is imagination, not seeing. Milton, as is well known, was blind, so the poem's repeated "sight" carries extra weight. In the dream he sees, which his waking life does not allow.
The turn comes late, at "But O" in line 13, and is brief. Almost the entire poem has gone to the dream's happiness, and the loss gets a line and a half. The last line is nearly all single syllables and quick clauses: "I wak'd, she fled, and day brought back my night." The verbs come like blows, with no time between waking and loss. Then the line turns the ordinary order of things upside down. Day, which should mean light, brings "back my night," so darkness is his waking condition and the dream was the only daylight he had. Given the poem's attention to sight, "night" can be read as his blindness, his grief, or both.
The poem is significant because it contains a great deal of feeling inside a strict form. It blends classical and Christian sources without seeming scholarly, because both are used for something personal. It also keeps its sorrow honest. The speaker has faith in a reunion in Heaven, but the sonnet ends on the empty morning, not on the promise. The hope is real, but it belongs to the future. The loss belongs to the present, and the poem lets the present have the last word.