| Quote #1
The trees are in their autumn beauty,
The language of the first four lines anticipates or foreshadows the sad tone that becomes more prominent as the poem proceeds. "October," "twilight," and "autumn" all describe literal things (a month, a season, a time of day) that are associated with darkness and gloom. As the poem proceeds, the speaker describes a more emotional kind of gloom.
| Quote #2
I have looked upon those brilliant creatures,
The speaker offers an interesting scenario here: after he looks on something "brilliant," he feels sad. It is probably because he realizes that he's changed, and so has the world, while these magnificent animals have not. The swans are still "brilliant," but life isn't so brilliant (people kill each other, people die, bad things happen).
| Quote #3
All's changed since I, hearing at twilight,
We get the sense that things are changed, and not for the better. It is interesting that the speaker separates the word "I" from "trod with a lighter tread." It is as if the speaker wants to avoid talking about how things used to be different (it makes him sad).