wishes to suggest that with the passage of time will come death. This motif also foreshadows the message of the following sestet.
In the third quatrain the speaker comments that if everything must grow old and wither, then the same must be true of beauty as well. The word beauty can mean the abstract quality of beauty. But the word is also a synonym for the young man. He is beauty, and he too will eventually die. His beauty will "forsake" (line 11) or leave him even if he is like a god of beauty.
In the couplet, the final two lines, the poet uses personification. The poet personifies Time as a figure that is carrying a "scythe" (line 13). A scythe is a farming tool that has a long, sharp blade attached to a long wooden handle. The farmer uses it to cut down crops, like wheat. In this poem, Time uses its scythe to cut down humans who are past their prime, who have grown too old. Traditionally in literature the dark figure carrying a scythe represents not Time, but Death. Of course, Shakespeare intentionally wants to connect the passage of time to death with this one image.
In the couplet the speaker also offers a solution to the problem he has presented, the problem of time being a destructive force. Thus, it would also be correct to view the structure of this sonnet as being divided between (1) the three quatrains, which introduce a problem, and (2) the couplet, which offers a solution.