In the beginning of scene 3, the Porter pretends to let people who have done wrong things for personal gain into the gates of "hell". What is the purpose of the porter in Act 2?

The goal of the porter is to serve as an alert to the reader that something terrible is going to happen in the castle.

Explanation:

When the porter refers to hell, he warns the reader that something big and so terrible is going to happen that can be compared to hell and all the wickedness and agony that this place symbolizes. This situation is the murder of the king in cold blood and the ususpation and persecution that the murderer will provoke, as well as all the mental lack of control and emotional exhaustion that will be presented during the narrative.

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explanation:

and also the mystery girl it might be someone that she might know in her school

the pronoun reference replaces a noun, but you have to make sure there's no room for confusion.

in the first sentence "her" could be referring to shely or to the sister. the answer there is "shelly called her sister and told her about the new baby."

in the second sentence "it" could be referring to the roof or the floor. the answer there is "since the roof leaks on the family room floor, we need to repair the roof."