Describe the changes of mood and atmosphere from the beginning of the act to the end of the act.
This certainly depends on the Act.
Act I opens up with feelings of havoc, fear, uncertainty, suspicion, and concern. It feels unsettled, like there is a big problem that truly needs resolution.
Throughout the middle of the Act, we see moments of the girls alone and conspiring. Within these moments, we see Abby controlling the others and trying to lie her way to a solution. For a few moments, the audience may believe she can pull it off.
By the end of Act I, the girls are naming good people in the town who many respect but they girls are acting as if they are conjuring spirits.
In the beginning of Act II, we are introduced to the marriage relationship between Elizabeth and John. It feels cold. They are in a struggling marriage, and for obvious reasons. You could cut the tension between the two with a knife. This is not a good place for marriage partners, but their Puritans so they have to deal with it.
As the Act continues, Mary Warren reveals what she knows and commits to go to court on behalf of the Proctors to tell the truth and clear all of this up. Once again, we feel a sense of relief. We believe the situation is going to be fixed.
By the very end, Danforth is giving orders and directing that Goody Proctor will have to come and testify. John gets defensive and demands Mary Warren's obedience. This feels like a lot of pressure for Mary because it pits her between her friends and her boss.
I don't want to give much more of the play away if you haven't read it. You can see how these first two acts go from a feeling of uncertainty to seeming resolution, and back to heightened struggle. This is how an author keeps you in their work. You want to know what's going to happen next.