Five years have past; five summers, with the length Of five long winters! (1-2)
The opening lines of the poem practically scream at you the importance of the past. The speaker repeats three times just how many years have gone by since he was last at the Wye: five. Five years.
The day is come when I again repose Here, under this dark sycamore, and view These plots of cottage-ground, these orchard tufts, (9-11)
Finally, after "five long winters," the speaker is able to repeat some of his earlier experiences. Or, at least, he's able to go over the same ground again. The experiences, as it turns out, are harder to duplicate.
These beauteous forms, Through a long absence, have not been to me As is a landscape to a blind man's eye (22-24)
The "beauteous forms" describe the lovely view. Even during his "long absence" from the Wye river valley, the speaker says that the "forms" themselves have been present to him. They haven't just been like an empty description, like a "landscape to a blind man's eye."
feelings too Of unremembered pleasure (30-1)
Remembering the "beauteous forms" of the Wye valley doesn't just conjure up memories of that particular nature walk. It helps remind the speaker of all the seemingly trivial "acts/ Of kindness" (34-5) that he's committed in his life.
And now, with gleams of half-extinguished thought, With many recognitions dim and faint, And somewhat of a sad perplexity, The picture of the mind revives again: (58-61)
Now that the speaker is back at the Wye again, he's "perplexed," or confused, at the disparity between what he remembers, and what he sees. His memory, which was so immediate during the "five long winters," has become "half-extinguished thought."
when thy mind Shall be a mansion for all lovely forms, Thy memory be as a dwelling-place For all sweet sounds and harmonies (139-142)
The speaker imagines Dorothy's mind and memory using house metaphors. Her mind will be a "mansion," and her memory a "dwelling-place" for all the beautiful things she sees and hears on this trip to the Wye valley. Then, in future years, if she's unhappy, she'll be able to revisit this mental "mansion" and see it all again in her mind's eye.