What happens in Act 1 of "The Crucible" to foreshadow the events that unfold by the end of the play?

There are several things that happen in Act One that, if you pay close attention, foreshadow some future events.  You have to pay close attention to what the characters say about each other, and the grievances and underlying bitterness that resides in everyone's tone and words.  First of all, pay attention to the bitterness and hatred that Abby expresses towards Elizabeth.  She calls Elizabeth and "gossiping liar," "cold," "hateful," and "lying, cold, sniveling."  Such intense hatred does not just go away.  We also learn that Abby is deeply in love with Proctor, and wants to continue their relationship. This, combined with her hatred for Liz, foreshadows Abby's future acts against Elizabeth Proctor.  It doesn't come as a surprise that she goes for her.

Next, pay attention to Mrs. Putnam.  She expresses intense grief and bitterness over the fact that she has lost all of her babies.  And, she seems to resent the fact that Rebecca Nurse has had lots of kids, and not lost one.  She exclaims bitterly to Rebecca,

"You think it God's workyou should never lose a child, nor grandchild either, and I bury all but one?"

She seems resentful towards Rebecca and her success as a mother, and resentful over Rebecca's mild self-righteous attitude.  This resentment comes into play later on, and is foreshadowed here in this act.

One other instance of foreshadowing is when Giles Corey speaks to Reverend Hale about the fact that his wife is reading strange books that, as he thinks, makes him unable to pray.  Hale takes note of this, as does everyone else in the room, and that tidbit of information about Martha Corey comes into play later on also; it foreshadows her future arrest.

The most blatant foreshadowing occurs when all of the girls start naming other women as witches, all to get out of trouble.  This foreshadows all of the insane accusations against hundreds of the townspeople that come in the later acts.

Act One is a great act in which Arthur Miller sets up the characters and lays the foundations for all of the future action, through their arguments, weaknesses and commentary.  If you pay close attention, you can be alerted to those instances of foreshadowing. I hope that those thoughts helped; good luck!