How does Abigail turn the court against Mary Warren in "The Crucible"?
When Mary first tells her story to the court, Danforth turns to Abigail and asks her if any of it is true. Abigail first protests against being questioned in this way, and then she openly threatens Danforth. She suddenly looks quite scared and says, "A wind, a cold wind, has come. Her eyes fall on Mary Warren." She goes cold, so that when Hathorne touches her, he confirms it, and she is "shivering visibly." The other girls -- Mercy Lewis and Susanna Walcott -- see what Abgail is doing and follow suit, saying how they freeze, too. The ploy works: Danforth turns to Mary and asks, in a very accusatory way, if she is witching Abigail and sending her spirit out to hurt her.
Later in Act Three, after Elizabeth Proctor has left the court, Abigail pretends to see a "yellow bird" upon the beam overhead, and she speaks to it as though it were Mary. She says, "But God made my face; you cannot want to tear my face. Envy is a deadly sin, Mary [....]. Oh, Mary, this is a black art to change your shape." Again, the other girls chime in to back Abigail up, and they all begin to repeat whatever Mary says, making it seem as though she is controlling them. Danforth asks Mary, "A little while ago you were afflicted. Now it seems you afflict others; where did you find this power? [....] You have seen the Devil, have you not?" Abigail has successfully cast doubt onto Mary's story and Mary herself.
In Act 3, Mary Warren is forced by John Proctor to come back to the court to tell the judges that the girls are lying and always have been and that everything that they are claiming has taken place never really happened. When Mary comes back to tell the judges this, they invite Abigail and the other girls into the courtroom. At this point, Abigail pretends to see a yellow bird on one of the rafters in the room. The other girls then pretend to see it as well. Abigail talks to the bird and asks it why it has come, begs it to leave her alone, and calls it "Mary" -- Abigail is trying to incriminate Mary Warren by making the judges think that she is sending out her spirit on Abigail and the girls in the form of a yellow bird that is trying to attack them. Unfortunately, the court does believe that there is an invisible bird and instead of getting herself in trouble, she sides with Abigail and calls John Proctor a witch.