The main example that Wharton uses is when Ethan describes the sledding hill for the first time and tells Mattie about how the tree could really do some damage, should anyone hit it.
Chapter 1: Ethan is on his way to pick up his wife’s cousin, Mattie Silver, who is attending a church dance. The night is cold;the village buried under snow. He stops for a moment to look over the long hill behind the church, a favorite place for coasters—a scene that foreshadows the tragedy of Chapter 9.
Chapter 2:As Ethan and Mattie walk home, they pass the big elm at the bottom of the hill, and Mattie mentions that her engaged friends, Ned Hale and Ruth Varnum, had almost run into it while coasting.
Chapter 3: Ethan recalls how sickly-looking Mattie had been when she arrived in Starkfield and how healthy and strong she has become--foreshadows her sickly future.
Chapter 4: Gives us more background on Ethan's mother who was energetic and chatty in her youth but became quiet and withdrawn after her illness. This also happens to Zeena within a year of their marriage.
Chapter 5: Ethan is able to savor his evening with Mattie, although Zeena’s absence causes some constraint. Mattie serves the pickles in one of Zeena’s best cut-glass dishes, and when the cat accidentally breaks it, both she and Ethan are terrified.
Two examples of foreshadowing that I remember in Ethan Frome would be when Ethan and Mattie were talking about a couple that was recently engaged and how they almost ran into a tree. This foreshadowed Ethan and Mattie's crash they have towards the end of the book. There is also when Ethan talks about how ill Mattie looked when she initial began to live with them and slowly she becomes radiant and full of life. This would foreshadows her permanent injuries that she has after the wreck.