William Shakespeare in "Sonnet 18"--"Shall I compare thee to a summer's day?"--speaks as the poet observing the beauty of the person to whom he is speaking, and inherent in this "speech" is the method of the poet and the poet's belief in the power of the poem he is writing. The sonnet begins as a question, noted in the first line quoted as a title above, and the poet follows the simile in this question with other imagery which compares the person to nature, to the best parts of the day and the best features of the natural world.
In structure, the first eight lines constitute one idea as the poet compares the person to the summer day and notes the different aspects of that day which illuminate how the poet feels about the person or which are offered as not coming up to the same level of beauty and vitality as the person:
Shall I compare thee to a summer's day? Thou art more lovely and more temperate. . . (1-2).
Having said this, the poet proceeds to show why the subject is more temperate, more on an even keel in terms of demeanor. The summer day is given to excessive heat, for instance, and also the summer is al too short. Its luster dims in time, giving way eventually to the fall, while the beauty and manner of the person who is being praised does not:
Rough winds do shake the darling buds of May, And summer's lease hath all too short a date. . . (3-4).
The poet expands on this idea by explaining further some of the reasons why summer is not as temperate as the person spoken to, for sometimes the sun is too hot and the weather too uncertain, and over time the complexion of the day may dim. Over time, the course of nature changes "untrimmed," with no way of stopping the change and retaining the beauty of the day.
In the last six lines, however, the poet reacts to this idea by showing that there is one way in which the ravages of time, the shifts that come from one season to another...