#1 "We walk to my grandfathers grave." -to- "Ten years ago I came here last." Walking to their grandfathers grave and then going into how he came here ten years ago is showing that the person preserved a memory of their grandfather and is switching from present to past.
#2 "Last summer." -to- "We can not find my grandfather's grave." Is going from past to present. The person is preserving a memory from last summer but can't find their grandfather's grave now.
#3 "The saphouse will shudder and slide
with the snow to the ground." -to- "Today
we take my grandfather’s last quart of syrup
upstairs." At first the person it telling us how soon the sap house will collapse after it starts to tell us how they are taking the syrup upstairs now. It changes from future to present.