Few of the events of Hamlet are foreshadowed in a straightforward way, which is striking because in Shakespeare’s tragedies, and especially in the tragedies which have a supernatural element (like the Ghost in Hamlet), the play’s climactic events are usually foreshadowed or even prophesied. The absence of foreshadowing helps create the sense that in Hamlet certainty is hard to come by, and it also raises the dramatic tension. Hamlet spends much of the play trying to decide whether or not to kill either himself or Claudius: if either of these deaths were explicitly foreshadowed, Hamlet’s deliberations would be less momentous.
Claudius’s death is partially foreshadowed by the Ghost. The Ghost is recognised by Barnardo as a “portentous figure” (I.i.), and Horatio agrees that it “bodes some strange eruption” (I.i.), but none of the characters who witness the Ghost in the opening scene are certain about what its appearance means. Hamlet, by contrast, jumps to the conclusion that the Ghost’s appearance indicates “foul play” (I.ii.) before he has even seen it, which may indicate the Ghost’s accusation that Claudius murdered Hamlet’s father is all in Hamlet’s head. Whether or not the Ghost’s story is a hallucination of Hamlet’s, Hamlet himself doubts whether the ghost is “an honest ghost” (I.iv). It could also be said that the Ghost does not foreshadow Claudius’s murder so much as cause it, so the exact relationship between the Ghost’s appearance and Claudius’s death is hard to pin down: the ghost’s appearance is part foreshadowing, part cause, and part red herring. In Hamlet, even messages from beyond the grave are hard to interpret and harder still to trust.
Horatio warns Hamlet that the Ghost “might deprive your sovereignty of reason/And draw you into madness” (I.iv.). The Ghost itself instructs Hamlet: “Taint not thy mind” (I.v.). These warnings foreshadow Hamlet’s descent into madness. However, as always in Hamlet, we see a further layer of complexity to the question of Hamlet’s madness. After his encounter with the Ghost, Hamlet tells Horatio that he may “put an antic disposition on” (I.v.), that is, pretend to be mad. The play therefore sets up two different ways to understand Hamlet’s increasingly erratic behavior: as the real madness predicted by the Ghost and Horatio, or as the “antic disposition” mentioned by Hamlet. This uncertainty makes Hamlet’s character ultimately mysterious.
Hamlet’s murder of Polonius is foreshadowed when Polonius tells the assembled court that he acted at university: “I did enact Julius Caesar. I was killed i’ th’ Capitol. Brutus killed me” (III.ii.). Shakespeare’s Julius Caesar was written at the same time as Hamlet, and very likely the actors who played Polonius and Hamlet in Hamlet would have played Caesar and Brutus in Julius Caesar. Contemporary audiences would have recognised the actors, and may have taken this line as a hint that Polonius faces the same end as Caesar. Even if audiences aren’t familiar with the actors or the plot of the play, Polonius’s line introduces the idea of the character being killed by a confidant. The foreshadowing of Polonius’s murder raises the tension in the scene which follows: Hamlet behaves more erratically than ever, and we realize that his behaviour may for the first time in the play have real consequences.