... don't look where they're going, she said, and we continued on from there as if nothing had happened.
* * *
That was in May. Spring has now been undergone.
The narrative structure in these chapters employs the rearrangement of chronological events to tell the narrative in non-sequential order. As the quote above from Chapter 25 shows, the prime element of chronological narrative rearrangement is the flashback.
A flashback is a literary device that looks back at a past event from a present moment. For instance, if you were to think, right now, of your sixth birthday, then write or speak about it, you would be employing flashback to give a narrative of a past event as seen from the present moment. Flashbacks are triggered or identified by past tense and/or by specific language in a pre- or a post-flashback position. Pre-position language might be, "I remember" or "it was different back then" or other expressions of this sort. Post-position language might be similar to what the Atwood used: "That was in May."
While Chapter 26 ends in present time musings, most of it is in flashback: "What? I said, when he didn't go on." Chapter 27 steps out of flashbacks and the past tense of most of Chapter 26 and continues in present time, "I walk with Ofglen along the summer street," though it slips easily in and out of small flashbacks:
Ofglen and I walk slowly today; we are hot in our long dresses, ... There used to be an ice cream store, somewhere in this block. ... We would go there, when she was little ...
Chapters 28, 29 and 30 follow the same structure. They are framed in present chronological time then slip into a series of flashbacks or an extended flashback. It is through this means of flashbacks interspersed with present narratorial time that the history leading up to the narrator's sojourn with the Commander is told. Yet through twists between times that occur between lines, the end of Chapter 30 weaves in and out of past tense and present tense to end with the narrator's present reality in the present time. Note the change in tense from "prayed" to "pray" "think" "keep":
What we prayed for was emptiness, ... I pray where I am, sitting by the window, ... I think about the chandelier too much ... How can I keep on living?