The first line of Shakespeare's Sonnet 18 appears to be a question:
Shall I compare thee to a summer's day?
Shakespeare doesn't ask, "May I," or "Can I," or "Would you mind if I," nor in any way does he ask for permission or even for acquiescence from the person to whom the poem is addressed, to compare them to a summer's day. Shakespeare instead employs a rhetorical question, which effectively turns the question into a statement.
In asking, "Shall I compare thee to a summer's day?" Shakespeare has already done it. He's already compared the person to whom the poem is addressed to a summer's day, at least in the abstract. He continues the comparison through the next seven lines by referring to and comparing the person to whom the poem is addressed to certain, specific aspects of a summer's day.
"Thou art more lovely and more temperate," Shakespeare writes, then lists the ways in which the person to whom the poem is addressed isn't like a summer's day.
Nothing that Shakespeare writes about in these eight lines expresses the true theme of the poem. The first eight lines are simply the preamble to the next four lines, in which Shakespeare moves a little closer to the theme.
The ninth line, "But thy eternal summer shall not fade," intentionally leads the person to whom the poem is addressed to ask the question, "Why not? Why won't my eternal summer fade?"
Three lines later, Shakespeare answers the question: "When in eternal lines to time thou grow'st." When someone writes a poem ("eternal lines") about you, Shakespeare says, speaking to the person to whom the poem is addressed, then your "eternal summer," your eternal youth, your eternal beauty can never fade.
In the concluding couplet, Shakespeare finally gets to the theme of the poem:
So long as men can breathe, or eyes can see,
So long lives this, and this gives life to thee.
The poem isn't about a summer's day or even about the person to whom the poem is addressed. The poem is about Shakespeare himself. Through "this," through my poem, Shakespeare says to the person to whom the poem is addressed, you are immortalized; and you, your youth, and your beauty will live forever.
The theme of Sonnet 18 is that poetry can immortalize people and qualities that are, in reality, only fleeting and ephemeral. The speaker in this sonnet declares that his lover is actually better than a summer day because they are lovelier and milder than such a day. The winds can blow too roughly and harm the new flower buds, and summer really does not last that long anyway. There is something about that particular season that makes it seem so short. Further, the sun can sometimes shine too hotly, and, then again, on the other hand, the sun’s light is sometimes dimmed by passing clouds. It seems that the beauty in nature is temporary only and that it cannot last forever.
However, the speaker...
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claims that his lover’s beauty, which should only last a similarly short time, will actually live forever in this poem. The beloved’s beauty will be as an “eternal summer” that never fades because this verse consists of “eternal lines” that will memorialize the beloved’s beauty forever. The speaker ultimately concludes that as long as there are people who are alive with the ability to read the words of the poem, the poem will be kept alive, as will the beloved one and their beauty.
The above commentators rightly argue that Sonnet 18 is about the eternity of Shakespearean “lines.” This interpretation, however, can be extended a little further. The sonnet is not only about Shakespeare’s “eternal lines,” but it is also about how in time Shakespeare’s observations grow. In the final line of the third quatrain Shakespeare notes “in eternal lines to time thou gorw’st.” The beloved’s beauty not only remains unchanged in perpetuity, but it also grows parallel to time. In the couplet Shakespeare confirms this observation: “this gives life to thee.” Shakespeare’s observations will not only be read “so long as men can breathe,” but they will also offer life to the object which Shakespeare appreciates and this life giving force will give the object the potential to live in perpetuity. As the object grows eternally Shakespeare’s observations too receives perpetual growth from within—critics even today continuously enrich Shakespearean “eternal lines” with new interpretations.
Nature fades but art is immortal. Though beautiful at moments in time, everything in nature enjoys but a moment of perfection. In time every virtue will be destroyed, every potential beauty ravaged by the elements, and every perfection will come to contain imperfections. In art, however, the essence of perfection will be captured. Though everything in the world dies and fades, the subject of poetry enjoys eternal life.
The general theme of the sonnet is that what is written about in poetry is eternal - specifically in this poem, Shakespeare is admiring a woman, and saying that her beauty will never fade because he is putting it into verse. He begins by comparing her to a summer day, and then saying she is much more beautiful. He continues comparing the details of the summer day to his subject showing how she is much fairer. He ends the poem by focusing on the subject, her beauty, and her qualities which will be made eternal by the poet putting them into poetry.
Sonnet 18 is of Shakespeare's most famous sonnets, and one whose themes and many quotes from it have been absorbed into the language, not least its famous opening line, "Shall I compare thee to a summer's day?".
It has many themes in common with Shakespeare's other sonnets, and I've put the themes in bold for you. It begins with comparing the beauty of a woman to the beauty of nature in detail, and despairing at the transitoriness and changableness of summer and nature (the cyclical, changing nature of nature). Even the sun, the speaker laments is changing - and he concludes that everything in the world changes:
And every fair from fair sometime declines,By chance or nature's changing course untrimm'd
However, the beauty of the woman will not fade - and, moreover, beauty will survive even death because the poet has preserved that beauty in writing.
The final couplet concludes that, as long as men can breathe or eyes can see, his writing can be read - and therefore the woman's beauty is eternal.
Hope this helps!