that time of year

That time of year thou mayst in me behold
When yellow leaves, or none, or few, do hang
Upon those boughs which shake against the cold,
Bare ruined choirs, where late the sweet birds sang.
In me thou see'st 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 deathbed whereon it must expire,
Consumed with that which it was nourished by.
This thou perceiv'st, which makes thy love more strong,
To love that well which thou must leave ere long.

One thing that has always stuck with me about this sonnet (by William Shakespeare) is the ambiguity I find in the last line. It's like a verbal optical illusion: is it inverted or not? Is 'thou' a subject (leaving) or an object (being left)? I'm sure it's for this reason that this sonnet has stuck with me from the first time I read it. Its undecidability means my brain will never put it down.