Let me not to the marriage of true minds
Admit impediments. Love is not love
Which alters when it alteration finds,
Or bends with the remover to remove:
O no; it is an ever-fixed mark,
That looks on tempests, and is never shaken;
It is the star to every wandering bark,
Whose worth's unknown, although his height be taken.
Love's not Time's fool, though rosy lips and cheeks
Within his bending sickle's compass come;
Love alters not with his brief hours and weeks,
But bears it out even to the edge of doom.
If this be error and upon me proved,
I never writ, nor no man ever loved.
The first rhetorical/poetic device that jumps out is Shakespeare's use of antanaclasis, which is repeating a word or phrase with a different meaning, as in "love is not love" (which changes when it gets the chance). The way he uses it here, it seems--at least at the end of line 2--to present a paradox: How can love not be love? Shakespeare uses enjambment, where lines are not end-stopped but flow onto the next line, to create momentary paradox or confusion (or as my Shakespeare prof put it long ago, "To make your mind turn cartwheels"). The confusion is quickly cleared up when he continues the thought on the next sentence, though.
The next two sentences use polyptoton, where one word is repeated in different grammatical forms. To wit: Love does not "[alter] when it alteration finds, / Or [bend] with the remover to remove." Alters/alteration and remover/remove are different uses of the same word. All of these are forms of repetition of basic ideas, but the use of the same word to mean different things or using the same stem word in different forms has a poetic, almost musical pleasure to it. It also forces your mind to pause and work out the nuances in meaning.
In the next four sentences, he uses metaphor to help describe what love is to him: it is "a star," which was used until very recent times to guide ships, an "ever-fixed mark" that is above worldly storms and is never shaken. It is, in other words, an unshakable thing, above petty troubles. True love, then, lies in the mind and does not "admit impediments" (allow obstacles to interfere).
Next, he personifies Love, saying it isn't "Time's fool" (thereby personifying Time, as well, as though the two are human adversaries). Time carries the proverbial sickle (a tool used to cut wheat) of Father Time, and Time can eventually take the "rosy lips and cheeks" of youth, but Love is not subject to this, either.
He continues the personification with the next two lines, saying that Love isn't affected by the passing of time; it remains constant--like the star he compared it to earlier--even unto "the edge of doom," or death.
Shakespeare is also big on alliteration, which is the repetition of (usually) consonant sounds in a sentence or more, usually at the beginning of words and emphasized syllables. Alliteration creates the effect of sound imitating ideas; it is music in words (think of it as another form of "rhyme," if you like). (For example, when Macbeth is talking about killing Duncan, he says, "If the assassination / Could trammel up the consequence, and catch / With his surcease success," he comes across sounding like the snake he is.)
Thus, when in Sonnet 116 Shakespeare writes "Let me not to the marriage of true minds / Admit impediments," he emphasizes the "m" sound. Not only does this have a pleasing, musical affect, but it is a sound we often associate with pleasure; when you take a bite of a delicious cookie, you probably say, "Mmmmm" (because your mouth is full, but it tastes good). He uses alliteration elsewhere in the poem, most notably when he speaks of how rosy lips and cheeks under "Time's bending sickle's compass come," the "k" sound almost onomatopoeic in how they cut the air.
The second line uses caesura, a complete pause in the middle of a line. The effect of this in music--and what is poetry but music with words?--is to produce a moment of anticipation, thus emphasizing what follows. Shakespeare thus pauses dramatically before he launches into his explanation of what love really is.
He also uses slant rhyme three times: love/remove, come/doom, and proved/loved. Slant rhyme is also known as "imperfect rhyme," among other things, and imperfection is part of the message: Shakespeare doesn't say or pretend that love is perfect--only that it rises above the petty problems couples have and is steadfast, even until death.
What are the writers tools in something 116