Milton's compliment to a composer is really an argument about how words and music ought to treat each other. The sonnet is addressed to "Harry", the Henry Lawes of the title, and it praises him for one specific skill. He "first taught our English Musick how to span / Words with just note and accent." To "span" is to measure, to stretch one thing exactly across another, and "just" means both exact and fair. Good setting, in Milton's eyes, gives each syllable the weight it already carries in speech. The musician serves the poem and does not drown it.
The poem also illustrates its own case. The opposite of Lawes is the listener with "Midas Ears", a reference to the myth of King Midas, who preferred the wrong music and was given donkey's ears as punishment. Such a person would "scan" lines badly, "committing short and long", and the verb makes a confusion of syllable lengths sound almost like a crime. Milton's pentameter is itself tidy enough to scan without trouble, so the sonnet does what it praises. The pun on "scan", a poet's word for measuring a line, ties the musician's craft to the poet's.
The octave rhymes ABBA ABBA, and its rhyme words almost make a miniature argument. "Song," "long," "throng" and "tongue" belong to one sound, while "span," "scan," "wan" and "man" belong to the other. The claim of line 8 comes where the two sets meet: Lawes "couldst humor best our tongue." The word "our" shows a small national pride. English, the poem implies, deserved music that understood it. Milton also lets himself speak with authority about the future. Lawes will "be writ the man" to "after age", and "writ" reminds us who does the writing. Envy, personified, goes pale ("wan") at so much praise, which suggests that the compliment is large enough to make rivals sick.
The turn comes at line 9, where the sestet moves from technique to reward. "Thou honour'st Verse, and Verse must send her wing / To honour thee." The repeated "honour" makes the exchange visibly mutual: the musician honours poetry, so poetry must honour him back. A manuscript variant reads "lend" for "send", which stresses the idea of one art lending its power to another. Lawes becomes "the Priest of Phoebus Quire." Phoebus is Apollo, a god associated with both music and poetry, so the two arts turn out to share a single patron. When Lawes "tun'st their happiest lines," he is also shown as a discerning reader who picks out the best poems to set.
The closing lines widen the view. Dante, the great poet himself, will "give Fame leave to set thee higher / Then his Casella." Casella is the musician Dante meets in Purgatory and "woo'd to sing." Milton makes Dante a kind of gatekeeper of reputation, and the honour for Lawes is to be ranked above the singer a great poet loved. The details are gentle: the "milder shades of Purgatory" suggest a place of waiting where a song can still bring pleasure. The final sentence unrolls across the last four lines and lands on the long, slow word "Purgatory," which rhymes quietly with "Story" in the sestet's pattern of CDE DCE. The rhymes link, in turn, "wing" with "sing" and "Quire" with "higher," so that flight, song and height all gather near the end.
The sonnet matters because it uses a compact form for a clear statement about the arts. Poetry and music are partners, and the best partnership happens when the musician is faithful to the poet's rhythm and sense. It also places an English composer in a European tradition by setting him beside a famous scene from Dante. And it shows how much can be packed into fourteen lines of praise, which carry classical myth, an argument about craft and a promise of lasting fame, without ever feeling like a lecture.