In The Crucible, why can't Mary Warren tell the truth in court?
Mary Warren finds herself in a very difficult position at the court. Proctor has brought Mary Warren there so that she can confess to the lies she had been telling along with Abigail and the other girls. If Mary Warren confesses, she will not only be telling the truth and saving her job. She will also be saving the life of Elizabeth Proctor.
Despite all she has to gain in telling the truth, Abigail shows Mary Warren how much she has to lose as well.
Mary Warren, the weakest of the girls and the most likely to be intimidated, finds she cannot stand up to the power of Abigail.
Threatened with having her fraud exposed, Abigail thinks quickly in court when Mary Warren tells Danforth that she had been lying in court when she said she saw spirits.
Abigail begins to "see a bird", or a spirit, near the ceiling. Abigail describes this spectre as threatening her and says that it is Mary Warren's doing. This action essentially functions as an accusation against Mary Warren, suggesting that Mary is using witchcraft.
When Abby turns against her and accuses her of sending her spirit out, she knows what will become of her.
Abigail is convincing in her act, along with the other girls, and Mary Warren recognizes that if she persists in telling the truth she will be accused of witchcraft outright. If she is accused and convicted, she will be put to death.
Her decision then becomes to either save Elizabeth Proctor with the truth or save herself with a lie. She chooses to lie.