The one word that is repeated in this excellent example of a Shakespearian sonnet is the word "time." Throughout the sonnets, and especially towards the beginning, Shakespeare attempts to describe the idealised beauty of the male object of the sonnets, though always referring to the fact that such beauty will wither and fade with the passing of time. Time is such a key theme of all of his sonnets that he either tries to immortalise his beloved's beauty in his verse or, as in this sonnet, advises his beloved to have children so as to perpetuate his beauty. Let us examine the use of the word time in this poem and see how it fits in with the overall purpose.
The first two lines of this sonnet present "time" as a negative thing, something that robs beauty and withers and decays:
When I do count the clock that tells the time,
And see the brave day sunk in hideous night;
It is "time" that transforms the "brave day" into "hideous night" and likewise is responsible for the passing of the seasons, which fills the world with signs of the death of nature. It is contemplating the havoc that time wreaks in nature that forces the speaker of the poem to realise that time, which is responsible for the decay and withering that he sees all around him, will have a similar effect on his beloved:
Then of thy beauty do I question make,
That thou among the wastes of time must go,
Since sweets and beauties do themselves forsake
And die as fast as they see others grow;
However beautiful the beloved is now, he, like everyone else, is destined to go "among the wastes of time." Note the negative way that time is presented here, linked to the word "wastes." Thus Shakespeare reinforces the effect of time on the human body.
Lastly, "Time" is personified as a Death-like figure, with a scythe, who is presented as all powerful and whom cannot be cheated, except by having children which would allow the beloved to create a temporary defence against the ravages of Time:
And nothing 'gainst Time's scythe can make defence
Save breed, to brave him when he takes thee hence.
Thus it is the word "time" that is repeated in this sonnet, which of course emphasises the central theme of the poem, which focuses on the ephemeral and fading beauty of the beloved and the impact of time upon us all.