How does Shakespeare use language for effect in Sonnet 18?

There are many ways in which Shakespeare manipulates language in Sonnet 18. The most obvious of these may be his extensive use of metaphor; that is, how youth and mortality are conveyed through natural motifs. Whereas "darling buds of May" (Line 3) and a lover who is "temperate" (Line 2) come to symbolize youth, "rough winds" (Line 3) and "shade" (Line 11) are associated with time, mortality, and death. Although the lover is like a spring day, s/he will eventually be "[shaken]" (Line 3) by the harsh reality of time. That is why the speaker decides to immortalize him or her in verse; in this way, the lover's "eternal summer shall not fade" (Line 9).

Another interesting use of language includes economic or financial metaphors. Indeed, the speaker notes that "summer's lease hath all too short a date" (Line 4), and that the lover will not lose all "possession" (Line 10) of his or her beauty. In this way, youth is compared to a commodity which is not infinite; ultimately, it will run out, and we will die. It is only through poetry that we have any chance of permanence in love. 

Thus, Shakespeare combines more esoteric metaphors (that is, from nature) with more practical or daily ones (that is, from finance). This clever intermingling of spheres adds to the uniqueness and power of the poem. Shakespeare reaches beyond the abstract, and brings his poetry into "real world" situations. 

We can also note that Sonnet 18, like all Shakespearean sonnets, is written in iambic pentameter, and consists of 14 lines. Its rhyme scheme (abab cdcd efef gg) creates musicality. Also, we should note that Sonnet 18 makes use of a volta, or turn; in the final couplet, the speaker reinforces his belief that love and poetry can be eternal ("and this gives life to thee" [Line 14]). This is just one example of how Shakespeare's structural choices enhance his poetic language.