How does the form of Shakespeare's Sonnet 18 "Shall I Compare Thee to a Summer's Day?" establish the theme? (What is the theme?)
Arguably his most well-known sonnet, Shakespeare's Sonnet XVIII is written in the Petrarchan sonnet form. This form lends itself well to the expression of the theme of the constancy of true love and its ability to be immortalized.
In his exultant first lines, the poem's speaker asks the rhetorical question, "Shall I compare thee to a summer's day?" and then contends that his lover is more "lovely and temperate" than the seasons. For "Rough winds" shake the delicate Spring buds from the trees, and in summer "too hot the eye of heaven shines." After these and other expansive reflections in the first eight lines, line nine contains the Petrarchan Volta, or turn in thought. It is here that Shakespeare begins his contrast of the lover's beauty with that of the summer's day: "But thy eternal summer shall not fade." The sestet of this Petrarchan sonnet then argues that unlike the temporal seasons' loveliness, the lover's beauty will be eternal because this beauty will be preserved in his verse as long as "men can breathe or eyes can see."
The Petrarchan sonnet is a poetic form that lends strength to Shakespeare's theme in Sonnet XVIII with the contrast between the two parts. For the question of his lover's being compared to a summer's day in the first eight lines is passionately answered in the final six lines, lines which express the theme of the poem. Perhaps partly because of its form, Sonnet XVIII is one of the most enthusiastic of Shakespeare's sonnets in its lovely poetic expression and passionate declarations.
I'm not sure what you mean by the term "form", but if what you mean is the content and how it implies the theme, then the first line, "Shall I compare thee to a summer's day" establishes the nature theme that prevails throughout the sonnet. This is because each line draws a comparison between the receiver of the poem and an element of nature. Take for example the next line, "rough winds do shake the darling buds of may". This line evidences the theme of nature established in the first line because it is again a comparison between the rough winds (an element of nature) to the receiver of the poem.