Final answer:
Shakespeare opens Sonnet 18 with a rhetorical question to engage the reader and introduce the theme of comparing the subject's beauty to the transient nature of a summer's day, which sets the tone for the poem.
Step-by-step explanation:
Shakespeare begins Sonnet 18 with a question, 'Shall I compare thee to a summer's day?', as a rhetorical technique to immediately engage the reader. The question is provocative and sets up a comparison that runs throughout the sonnet, drawing the reader into a reflection on the nature of beauty and its permanence. This opening line is a critical element of the poem's structure, working to both capture the reader's attention and introduce the theme of comparing the subject's beauty to a summer's day, only to find that the subject surpasses this comparison.
Furthermore, the use of rhetorical questions in poetry can establish tone, theme, and provoke deep contemplation, and in the case of Sonnet 18, presents the poet's argument that the subject's beauty will be eternal through the preservation of the sonnet itself.