I need help with this question: “...Salem folk believed that the virgin forest was the Devil’s last preserve...” How is the forest used to...

Miller tells us early on, in the exposition that precedes act one,  

The edge of the wilderness was close by.  The American continent stretched endlessly west, and it was full of mystery for [the Puritans].  It stood, dark and threatening, over their shoulders night and day, for out of it Indian tribes marauded from time to time, and Reverend Parris had parishioners who had lost relatives to these heathen.  

For the Puritans, the forest...

Miller tells us early on, in the exposition that precedes act one,  

The edge of the wilderness was close by.  The American continent stretched endlessly west, and it was full of mystery for [the Puritans].  It stood, dark and threatening, over their shoulders night and day, for out of it Indian tribes marauded from time to time, and Reverend Parris had parishioners who had lost relatives to these heathen.  

For the Puritans, the forest was a lawless place, a place that existed frighteningly outside of the rules of law and order that gave them some sense of control in their tiny community.  One reason they thought of the forest as a place out of bounds had to do with the fact that, when an Indian tribe raided a Puritan settlement, the Indians always came out of the woods.  Although, these raids were not an everyday occurrence, they were common enough that most people knew someone whose family had been affected by one.  To the Puritans, the brutal and savage Indians were doing the Devil's work.

In addition, as Reverend Parris tells Abigail, "Abominations are done in the forest."  The forest is a place where people can go to escape rules; they can hide within the trees and engage in whatever terrible or immoral behavior they choose.  This might be a gathering of witches dancing naked and conjuring spells or an unmarried couple retreating to the darkness of the forest in order to have their affair.  The temptation presented by the forest was very much believed to be the handiwork of the Devil.  It is the Devil who tempts us to do bad things, and bad things are easier to do in the forest.