In The Crucible, why is Rebecca Nurse in jail?
In Act II, Scene 3 Giles Corey and Francis Nurse arrive at Proctor's house and inform John, Elizabeth, and Hale that their wives (Martha Corey and Rebecca Nurse) have been put in jail. Their incarcerations are related to the contagious accusations of witchcraft and witchcraft-related offenses. Martha Corey has been arrested for putting a curse on a farmer. Rebecca has been charged with killing Goody Putnam's babies. Goody Putnam notes, in Act I, that she's lost seven children in childbirth. Thus, she's always suspected some deeper reason, perhaps a supernatural one, as to why she's had such misfortune.
In Act I, Mrs. Putnam reveals her frustration and suspicion (of Rebecca, the supernatural, etc.) as to how someone (Rebecca) can never lose a child and she, Goody Putnam, can lose so many:
You think it's God's work you should never lose a child, nor grandchild either, and I bury all but one? There are wheels within wheels in this village, and fires within fires!
Mr. Putnam then tells Parris to look for signs of witchcraft when Hale arrives. Many women in Salem had been accused of witchcraft for an assortment of reasons, but the presumption here is that the Putnam's accused Rebecca Nurse.
Rebecca Nurse is in jail because she has been accused of witchcraft. In Act Two, when Giles Corey and Francis Nurse arrive at the Proctors' home, Francis tells John Proctor and Mr. Hale that Cheever came to their home and took Rebecca away in his wagon (the wagon which will very soon arrive to cart Elizabeth Proctor away as well). Francis tells the crowd that the arrest warrant read, "For the marvelous and supernatural murder of Goody Putnam's babies." Therefore, it is implied the Putnams have accused Rebecca of murdering their children.
In Act Four, Rebecca remains in jail because she has been convicted of witchcraft but refuses to confess. A confession would actually mean that the accused would have some jail time but would not, ultimately, be hanged for his or her crimes. Hanging was reserved for those convicted who refused to confess. Rebecca is only brought from the jail at this point because she is schedule to be hanged.