Hello NHD,
A sonnet is a highly constrained form of poetry, as it must be written in accordance with various "rules" (most notably, there should be 14 lines, and the poet should employ one of a handful of rhyme schemes).
This sonnet however is quite irregular; among other things, Keats uses the irregularity to exemplify the suggestion in the text – namely, that if poetry is to be constrained by the sonnet form, poetic considerations should nonetheless outweigh merely formal considerations.
(It might be noted however that the poem itself is a tissue of very familiar and well-worn images. While the form may be novel, the content is not.)
MrP
