Usually, when one thinks of the use of the literary technique called "foil," it is associated with determining opposite traits between two characters. Foil characters help to compare and to contrast between the two, whether it be physically, emotionally, intellectually, or otherwise. Sometimes foil characters are complete opposites in moral standing, but they equal each other in power or energy. Then, these two foil characters are pitted against one another to see what would happen in a battle and who would come off conqueror.
Here, however, the question asks how the nighttime forest and the daytime in a colonial Puritan village might be foils. Comparatively, both the forest and the village are filled with people whom Young Goodman Brown knows; all neighbors, friends, and family end up in the forest at some point during the night, while they also live in the village during the day. The difference is, though, that during the nighttime in the forest, people aren't who they say they are. They don't act like they normally do during the day. In contrast, of course, the nighttime forest brings with it darkness, mystery, and confusion until everyone is revealed to have a part in dark journey. Young Goodman Brown is demoralized as he realizes that nothing is as it seems during the light of day and that none are immune to the temptations of sin (represented by the dark forest and the activities found therein). Therefore, the nighttime forest in the short story of Young Goodman Brown serves as an equal, yet opposite, setting for daytime in the village.
Brown returns home to his wife, Faith, eternally stained and dejected by the night's revelations. He is never the same, even though he vowed to himself "after this one night, I'll cling to her skirts and follow her to heaven."