Sonnet 18: an analysis of the rhymes
Shall I compare thee to a summer's day?
Thou art more lovely and more temperate:
Rough winds do shake the darling buds of May,
And summer's lease hath all too short a date:
Sometime too hot the eye of heaven shines,
And often is his gold complexion dimm'd;
And every fair from fair sometime declines,
By chance, or nature's changing course, untrimm'd;
But thy eternal summer shall not fade
Nor lose possession of that fair thou ow'st;
Nor shall Death brag thou wander'st in his shade,
When in eternal lines to time thou grow'st;
So long as men can breathe or eyes can see,
So long lives this, and this gives life to thee.
Commenting on the effects of a technique often seems to people to be one of the most difficult things to do when discussing poetry, but if you think of the technique in relation to the effect of the poem it will become easy!
If we think about the tone, or atmosphere, of a poem, it will help us to comment on the technique. This is because a good poet will combine techniques in such a way as to build up the atmosphere. In this case, the poem is about the perfection of the person addressed (by the way, it is generally agreed that it was written to a man, not to a woman!), and the point of the poem is to say that this person's perfection lives forever in the poem itself.
By making this point, Shakespeare is more or less saying, "Look how perfect my poem is! That's how perfect this person is!" So this sonnet is full of the poet's confidence in his own power to create something beautiful and perfect, something that will be worthy of the perfect person he is writing about.
In that case, I can suppose the rhymes help to create that sense of confidence and perfection that runs through the poem. So I might notice the way "buds of May" is tied to "summer's day" partly through the rhyme, but also through the imagery of summer and flowers in bud, giving a sense of something wonderful.
All the time, though, we are aware that summer, though wonderful, has its weak points; though the sun "shines", like everything else in nature it "declines"; the rhyme holds those two things together, and the heaviness of "dimm'd" (the image of a cloudy day) is linked to "nature's changing course untrimm'd" (the idea that things must inevitably change and lose their perfection over time).
So far then, the rhymes ("summer's day", "buds of May") have helped to create a sense of how wonderful summer is, but even such a wonderful thing has its weaknesses; it "shines" but is "dimm'd"; it "declines" as part of nature's inevitable "untrimm'd" process. The rhyming words reinforce the thought content of the poem.
Then Shakespeare picks up negative ideas - "fade", linked to death's "shade" - but turns them into a double negative - "not fade", "Nor" wander "in his shade"; the negative idea qualified negatively (by "not" and "Nor") makes a positive. Added to this are the positive ideas of "that fair thou ow'st" ("ow'st" here meaning "ownest", i.e., have or possess) and "to time thou grow'st". Again, the linking of these positive qualities through the rhyme reinforces the poet's point, which turns out to be, not so much the perfection of the person addressed, but the perfection of "this" - the poem itself; eyes will always "see", and so this poem will always give life to thee". The final rhyming couplet gives the poem its triumphant conclusion.
The first step is to identify the technique. I hope that, as a student of literature at this university, you will develop the ability to describe the effect of the technique.