1. The Ghost of King Hamlet: The audience is aware that the Ghost of King Hamlet is indeed the spirit of his father and not a creation of Hamlet's imagination. The audience is able to see and hear the Ghost while other characters like Claudius and Gertrude do not.
2. Claudius's guilt: The audience knows that Claudius, the new king and Hamlet's uncle, is guilty of murdering his brother, King Hamlet, to seize the throne. Claudius's confession to the audience in the soliloquy "O, my offense is rank" reveals his guilt and inner conflict.
3. Gertrude's ignorance: The audience knows that Queen Gertrude is unaware of Claudius's involvement in King Hamlet's death. She is initially deceived by Claudius's lies and believes him to be a loving husband. However, the audience is privy to the truth and can empathize with Hamlet's struggle as he tries to reveal the deceit to her.
4. Polonius's deception: The audience is aware that Polonius, the Lord Chamberlain, is employing deceit and manipulation to advance his own agenda. He spies on his son Laertes and coerces Ophelia to use love as means of surveillance of Hamlet.
5. Ophelia's state of mind: The audience can witness the deterioration of Ophelia's mental state as she comes under immense emotional strain due to her father's death and Hamlet's rejection. The audience can comprehend her state of fragility even before it becomes evident to the other characters in the play.
These are some of the things that the audience knows to be true in the play "Hamlet." This knowledge creates dramatic irony, generating tension and suspense as the characters navigate their own understandings and misunderstandings of events.