The trees are in their autumn beauty, The woodland paths are dry, (1-2)
While the poem wants to make nature seem like a place that is unchanging, these lines complicate that idea. "Autumn," like spring, is a season associated with change (i.e., the end of summer and the coming of winter).
Under the October twilight the water Mirrors a still sky (3-4)
The "still sky" and the water that reflects it give an image of tranquility. The word "still" is important because it suggests that nothing is moving or changing. There is sense of permanence to this scene.
Upon the brimming water among the stones Are nine-and-fifty swans (5-6)
The speaker waits until the last possible moment to mention the swans. "Swans" is the last word of the first stanza. Still, they seem to dominate his attention. You'd have to focus pretty hard to count fifty-nine separate swans (they don't exactly line up to be counted, you know).
I saw, before I had well finished, All suddenly mount And scatter wheeling in great broken rings (9-11)
The swans "suddenly mount" and "scatter." The swans abandon the speaker, in a way that foreshadows his vision at the end. He's doomed to be abandoned by these swans (as he sees it), but they'll be moving on, unchanged, to impress some other folks.
I have looked upon those brilliant creatures, And now my heart is sore (13-14)
Nature, in all its beauty, makes the speaker feel sad. Or maybe it doesn't. Just because he says "and" doesn't mean the image of the swan causes him to be sad. Maybe he just means "Now, at this moment, my heart is sore, even though I have looked upon those brilliant creatures." He may be ten times more depressing without these swans around!
Unwearied still, lover by lover, They paddle in the cold Companionable streams or climb the air; (19-21)
The swans are "unwearied still." Despite everything that has happened in the world, they remain unaffected. While this seems enviable (they just go on paddling), it's also kind of a problem. They are "still," a word that here and elsewhere in the poem reminds us of a lack of change, but also of a lack of growth and development.
But now they drift on the still water, Mysterious, beautiful (25-26)
The speaker describes nature as something we cannot fully understand. We know that it is beautiful, but it is still mysterious. In other words, it isn't like a math problem that we can quite "solve." Perhaps the mystery of the swans partly accounts for their power over the speaker's psyche.
[…] when I awake some day To find they have flown away? (29-30)
Here we get a sense of the potential alienation of man from nature. The speaker imagines the swans leaving him again, flying away and setting up shop somewhere else. It seems that the swans, like the rest of nature, will outlast him in the end. Sad.