Keats' poem is a Shakespearean Sonnet with an elevated tone and is divided into three quatrains and rhyming couplet as opposed to octave and sestet. Continuity is gained by the repetition of the word "when" at the beginning of each quatrain. This builds the tension of the poem describing areas of concern for unfinished tasks. The word "before" in the second line is echoed during the third reinforcing the speaker's list of desired accomplishments before it is too late. This syntax sets the tone to ensure understanding of the speaker's growing fear about time running out.
The repetitive use of the word "when" gives momentum throughout the narrative and serves to signpost the changing subject matters using strong imagery for each metaphor. In the first the speaker likens the literature he has yet to write as "the full ripened grain". In the second he looks to the sky with "huge cloudy symbols" to describe loves not yet found and finally "fair creature of an hour" to illustrate the passing of time.
The repetition of the word "never" is another reference to the passing of time. The suspensory pauses draw us forward at the enjambments on the third line of every quatrain stressing the urgency to avoid missed opportunities.
The summing up of the sonnet starts when the rhythm is broken midway in line 12 using a caesural pause at the word "then" before commencing the closing rhyming couplet. An emotional pause at the words "stand alone" leads us to fully comprehend the speaker's plight with the final line where "love and fame to nothingness do sink". The rhyme scheme (ABAB CDCD EFEF GG) is known as a Shakespearean method.
In conclusion, using repetition keeps the continuity of thought throughout the sonnet and the meaning is conveyed using strong imagery. The sonnet builds to its climax leaving the reader with the sad realisation that the speaker's ultimate fear is actually founded.