[Line 1]* - 'that time of year' being late autumn or early winter.
[Line 2]* - Compare the line to Macbeth (5.3.23) "my way of life/is fall'n into the sere, the yellow leaf".
[Line 4]* - 'Bare ruin'd choirs' is a reference to the remains of a church or, more specifically, a chancel, stripped of its roof and exposed to the elements. The choirs formerly rang with the sounds of 'sweet birds'. Some argue that lines 3 and 4 should be read without pause -- the 'yellow leaves' shake against the 'cold/Bare ruin'd choirs' . If we assume the adjective 'cold' modifies 'Bare ruin'd choirs', then the image becomes more concrete -- those boughs are sweeping against the ruins of the church. Some editors, however, choose to insert 'like' into the opening of line 4, thus changing the passage to mean 'the boughs of the yellow leaves shake against the cold like the jagged arches of the choir stand exposed to the cold'. Noted 18th-century scholar George Steevens commented that this image "was probably suggested to Shakespeare by our desolated monasteries. The resemblance between the vaulting of a Gothic isle [sic] and an avenue of trees whose upper branches meet and form an arch overhead, is too striking not to be acknowledged. When the roof of the one is shattered, and the boughs of the other leafless, the comparison becomes more solemn and picturesque" (Smith 148).
[Line 7]* - 'black night' is a metaphor for death itself. As 'black night' closes in around the remaining light of the day, so too does death close in around the poet.
[Line 8]* - 'Death's second self' i.e. 'black night' or 'sleep'. Macbeth refers to sleep as 'The death of each day's life' (2.2.49).
[Line 12]* - 'that' i.e. the poet's desires.
[Line 13]* - 'This' i.e. the demise of the poet's youth and passion.
[Line 14]* - 'To love that well '. The meaning of this phrase and of the concluding couplet has aroused much debate. Is the poet saying that the young man now understands that he will lose his own youth and passion, after listening to the lamentations in the three preceding quatrains? Or is the poet saying that the young man now is aware of the poet's imminent demise, and this knowledge makes the young man's love for the poet stronger because he might soon loose him? What must the young man give up before long -- his youth or his friend? The answer could lie in the interpretation of both the young man's and the poet's character in other sonnets.
Sonnets 71-74 are typically analyzed as a group, linked by the poet's thoughts of his own mortality. However, Sonnet 73 contains many of the themes common throughout the entire body of sonnets, including the ravages of time on one's physical well-being and the mental anguish associated with moving further from youth and closer to death. Time's destruction of great monuments juxtaposed with the effects of age on human beings is a convention seen before, most notably in Sonnet 55.
The poet is...