SONNET LXXIII
This sonnet is part of the collection composed by 154 sonnets of Shakespeare.
Just considering the lay out you understand that is a sonnet organised by the Shakespearean structure of three quartrains and a couplet.
In the first quartrain the poet addresses to his unknown love who he just calls 'thou', and says to him that he is getting old. He did this through a metaphor between himself and autumn: his body, indeed, is like a tree which lost leaves " When yellow leaves, or none, or few, do hang".
In this sonnet as in the XVIII one, the poet uses the semantic fields of nature and time. You can see the nature one in the first quartrain, and the semantic field of time in the second, when the poet says to the fair youth that he is getting to die, through the metaphor of time " In me thou seest the twilight of such day, as after sunset fadeth in the west, which by and by black night doth take away, death's second self, that seals up all in rest." (Lines 5,6,7,8). The start of the third quartrain is the same of the second one to creating collusion. He is an another metaphor 'That on the ashes of his youth doth lie' (line 10), to express that the youth is leaving.
In the couplet the poet invites his love (but also the readers) to take care of his/their lovers, because time passes soon. And the poet says that as time passes, love increases. 'This thou perceiv’st, which makes thy love more strong,
To love that well, which thou must leave ere long' ( lines 13-14)