That time of year thou mayst in me behold, (1)
Shakespeare draws parallels between humans and the natural world from the very beginning of the poem—and, as you can see here, invites us, the readers, to "behold" these connections. Why do you think he insists on these parallels so much? Could it be a way of emphasizing the inevitability of the processes of aging?
When yellow leaves, or none, or few, do hangUpon those boughs which shake against the cold, (2-3)
In these lines, the speaker continues the initial parallel he drew in line 1, comparing himself and his position in life to a tree just before winter. Because everybody in Shakespeare's initial audience would have known what winter feels like (Shakespeare probably wasn't writing with readers from equatorial regions in mind), this parallel between human experience and the natural world would have communicated instantly what the speaker was getting at. All you folks from Maine know what we're talking about.
Bare ruined choirs, where late the sweet birds sang. (4)
Something weird is definitely going on here—it's as if the speaker's general metaphor has doubled back on itself. Now, all of a sudden, he is describing nature in human terms, comparing the boughs of the tree to the wooden "choirs" (i.e. choir benches in a church) "where late the sweet birds sang." So which way is it, Mr. Shakespeare-man: are humans like nature, or is nature like humans? Why do you think the poet would want to mix things up like this? Could he just be showing off?