When you read "Tintern Abbey," you might think that it's prose cleverly disguised as poetry. You hardly notice that it's in verse (see "Form and Meter" for more on that). It sounds like the speaker is musing to himself, going over the details of the scene to re-familiarize himself with it after having been gone a long time. It's like what you might think to yourself if you go to visit the house you grew up in after your first year of college: "Oh, look, there's smoke coming out of the chimney; the neighbors must have a fire going. Wow, my parents really changed up the décor in the kitchen…and…hey! They turned my room into a home office!"
But then, the speaker hits a few snags, too. First of all, he doesn't remember everything looking quite exactly the way it does now. But he's not sure if he's changed, or if the scene has. Again, it's like going back to the house you grew up in. When the speaker starts getting philosophical ("did I get taller, or did the house get smaller?"), he also starts using much more dramatic, traditionally poetic language, like "thee" and "thou."