One of the best known of Shakespeare's sonnets, Sonnet 18 is memorable for the skillful and varied presentation of subject matter, in which the poet's feelings reach a level of rapture unseen in the previous sonnets. The poet here abandons his quest for the youth to have a child, and instead glories in the youth's beauty.
Initially, the poet poses a question — "Shall I compare thee to a summer's day?" — and then reflects on it, remarking that the youth's beauty far surpasses summer's delights. The imagery is the very essence of simplicity: "wind" and "buds." In the fourth line, legal terminology — "summer's lease" — is introduced in contrast to the commonplace images in the first three lines. Note also the poet's use of extremes in the phrases "more lovely," "all too short," and "too hot"; these phrases emphasize the young man's beauty.
Although lines 9 through 12 are marked by a more expansive tone and deeper feeling, the poet returns to the simplicity of the opening images. As one expects in Shakespeare's sonnets, the proposition that the poet sets up in the first eight lines — that all nature is subject to imperfection — is now contrasted in these next four lines beginning with "But." Although beauty naturally declines at some point — "And every fair from fair sometime declines" — the youth's beauty will not; his unchanging appearance is atypical of nature's steady progression. Even death is impotent against the youth's beauty. Note the ambiguity in the phrase "eternal lines": Are these "lines" the poet's verses or the youth's hoped-for children? Or are they simply wrinkles meant to represent the process of aging? Whatever the answer, the poet is jubilant in this sonnet because nothing threatens the young man's beautiful appearance.
Then follows the concluding couplet: "So long as men can breathe, or eyes can see, / So long lives this, and this gives life to thee." The poet is describing not what the youth is but what he will be ages hence, as captured in the poet's eternal verse — or again, in a hoped-for child. Whatever one may feel about the sentiment expressed in the sonnet and especially in these last two lines, one cannot help but notice an abrupt change in the poet's own estimate of his poetic writing. Following the poet's disparaging reference to his "pupil pen" and "barren rhyme" in Sonnet 16, it comes as a surprise in Sonnet 18 to find him boasting that his poetry will be eternal.