Sonnet 73

 1. The three things that speaker says that you can see in him or her are the twilight of such day, glowing of such fire, and his ashes of his youth doth lie/old age. 

 2. The last two lines of the poem are "This thou perceiv'st, which makes thy love strong. To love that well, which thou must leave ere long." This means to me that you have to recognize those things to strengthen his love with knowledge that he soon will be left alone because the speaker's fire will be gone. 

 3. If I were to insert line breaks into this poem, I would insert them under every fourth line of the sonnet. So then the poem would now appear:
  
 "That time of year thou mayst in me behold
When yellow leaves, or none, do hang
Upon those boughs which stake against the cold,
Bare ruin'd choirs, where late the sweet birds sang.

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. 

In me thou see’st the glowing of such fire 
That on the ashes of his youth doth lie, 
As the death-bed whereon it must expire 
Consumed with that which it was nourish’d by. 

This thou perceivest, which makes thy love more strong, 
To love that well which thou must leave ere long. "

I would divide the poem like this because each stanza is built upon each other, and when moved to the next stanza it kind of moves to a next description. It helps me to understand the poem to better sense than if it were just all one "stanza." The last stanza wraps up the whole poem, kind of like a summary of a story.