In Sonnet 18, Shakespeare compares the beloved's beauty to a summer's day by explaining how he's livelier and warmer than summer and that, even though summer will eventually pass, the beloved's beauty won't ever fade, as the poem will celebrate it forever.

In "Sonnet 18," the poet identifies three problems with a summer's day: it is too severe, with harsh temperatures and unpredictable weather; it can be too rough, as rough winds shake the delicate buds of May; and it is too brief, with summer's lease being all too short. In contrast, the poet's beloved is more temperate, lovely, and enduring.

The poet's beloved is made eternal through the enduring nature of the poem itself. In "Sonnet 18," the speaker asserts that while human life is fleeting, the verses he writes will preserve his beloved's memory forever. The poem's lines will live on as long as people can read or hear them, thus granting immortality to the beloved through the lasting power of Shakespeare's words.

Lines 7-8 of Sonnet 18 suggest that all beauty eventually fades, either due to chance or the natural passage of time. This decline in beauty is inevitable for everything in nature. However, the poem goes on to assert that the lover's beauty will remain eternal, preserved through the "eternal lines" of the sonnet itself, ensuring that as long as people read these words, the lover's beauty will endure.

The eternal summer in "Sonnet 18" won't fade because Shakespeare immortalizes his love's beauty through his poetry. While a summer day will eventually end, the subject's beauty will remain eternal as it is captured in Shakespeare's verse. As long as people can read his sonnet, her beauty will endure, ensuring she lives on forever through his words. This reflects Shakespeare's confidence in the lasting power of his poetry.

In "Sonnet 18," Shakespeare argues that beauty evades death through the immortality provided by poetry. The poet suggests that while natural beauty fades due to time or chance, the subject's beauty will endure in the "eternal lines" of the sonnet. The poem itself ensures that the subject's beauty will never fade nor be claimed by death, as it will live on as long as people read the sonnet.

The speaker's purpose in writing his "eternal line" is to immortalize the subject by metaphorically grafting their life to eternity through his verse. This requires time to continue and people to read his lines; otherwise, the metaphor loses significance. The "eternal line" serves to create a lasting rhythm, intended to be spoken aloud, ensuring the subject's enduring legacy through the enduring nature of the poetry itself.

In lines 7-8 of Sonnet 18, Shakespeare suggests that beauty can decline due to "chance" or "nature's changing course untrimmed," implying that external circumstances or the natural aging process can diminish beauty. However, the speaker asserts that unlike nature's transient beauty, the beloved's beauty is eternal, immortalized through the poem itself. Thus, while natural beauty fades, the beloved's beauty remains constant in verse.