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).