A critical analysis of Shakespeare's Sonnet 18 discusses everything from structure to rhetorical figure of speech word schemes. The structure is that of an English, or Shakespearean, 14 line sonnet having three quatrains with one ending couplet. This differs from the Italian Petrarchan sonnet form of two quatrains and one sestet with no rhyming couplet. The rhyme scheme is the traditional English sonnet scheme of abab cdcd efef gg. The underlying metaphor is built upon a comparison of his beloved's youth and beauty to a summer's day. The poetic speaker asserts that she cannot be thus compared because she shall be eternal through the power of his poetic lines.
The first quatrain (lines 1-4) says she is more lovely than a summer's day and more "temperate" than the "darling buds of May": so the summer day and she are contrasted with each other. The second quatrain says summer days can be too hot, decline, be dimmed and changed: "fair from fair sometime declines." The third quatrain says that she will not fade nor know death like a summer day will do because she will continue "in eternal lines to time." The ending couplet finalizes the theme of eternal beauty and youth caught in the poet's immortalizing lines by saying she will live as long as "men can breathe or eyes can see." The theme can thus be stated as: Eternal beauty and youth are bestowed by the poet's immortal and immortalizing lines that withstand the diminishment of time, quite unlike "a summer's day."
The structure adheres to the sonnet form that specifies a problem or complication be given in the second quatrain (lines 5-8) to the situation introduced in the first quatrain (lines 1-4). In this case, the problem (5-8) is that summer days are diminished and so is mortal beauty. The situation (1-4) is the contemplation of youth and beauty in comparative relation to a summer's day. The third quatrain (lines 9-12) offers the solution or resolution to the problem. (The subject change at line 9 is called the volta, or turn.) In this case, the solution (9-12) is to make her "eternal summer" of youth and beauty immortal through lines of poetry. The ending couplet finalizes the thought of lines 9-12 and makes the concluding pronouncement. In this case, it is:
So long as men can breathe or eyes can see,
So long lives this and this gives life to thee.
Poetic techniques Shakespeare uses include metaphor (e.g., "too hot the eye of heaven"; "his gold complexion dimmed"); personification (e.g., "shall Death brag"); and his trademark word play where varying meanings of a word are played off of each other (e.g., "every fair from fair"). Some key rhetorical techniques used as figurative word schemes are hyperbole (e.g., "So long as men shall live"); polysyndeton, which is the use of "and" for rhetorical effect (e.g., "So long lives this and this gives life to thee"); and chiasmus, which is inverted parallelism also as in "So long lives this and this gives life to thee." The inversion is as this illustrates: lives ==> this / this ==> gives.