"In Xanadu did Kubla Khan" (line 1)
When does this poem take place? Here we seem to be in the past, when the actual Kubla Khan built his palace, in the 13th century. The dream mentioned in the subtitle is happening in the present, though, and it's an experience that the speaker is having. Later it seems that he is looking into the future. We aren't looking for one answer here, but keep an eye on the way in which this poem marks the passage of time.
"Ancestral voices prophesying war!" (line 30)
The idea of history keeps coming up here. In this line Kubla, who is in the past, is remembering an even deeper past. But the people in the past are "prophesying" or imagining the future. Confused? That seems like part of the idea. In a vision or a dream, time can take on strange twists, and appear to be different from what we are used to.
"In a vision once I saw" (line 38)
Here the "once" tells us that we are moving into a new time frame. This is a big shift for the poem. We were dealing with one vision, and now we've got another one on our hands. Keep an eye on these time changes, since they are important markers.