"Is this a dagger which I see before me,
The handle toward my hand..."
See in text (Act II - Scene I)
This quote illustrates foreshadowing (a clue as to what may come). Macbeth "seeing" the dagger foreshadows the manner by which Duncan will die and Macbeth's coming hallucinations. The bloody dagger becomes a symbol for Macbeth's rampant ambition.
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"so, it will make us mad...."
See in text (Act II - Scene II)
These words are filled with dreadful irony and foreshadowing. Considering how Macbeth has already imagined a floating dagger prior to killing Duncan and his current agitated state, the likelihood that one or both of them will mentally suffer as a consequence of this action is a distinct possibility.
"Macbeth shall sleep no more..."
See in text (Act II - Scene II)
Given the supernatural elements present in this play, it is impossible to say for certain whether Macbeth hears or imagines this voice. However, its words accurately foreshadow his fate. The insomnia Macbeth suffers due to his guilt will contribute to a blurring of the imaginary and the supernatural, as he continues to perceive sounds and visions that may or may not be real.
"Knock, knock! Who's there, in th’ other devil's name?..."
See in text (Act II - Scene III)
Knocking is a motif throughout Macbeth. After Macbeth and his wife complete their hideous murder, they hear a knock within that causes them to immediately fear and begins their slow descent into guilty madness. The Porter imagines Hell's Gate and unwittingly invokes two symbols. First, Macbeth's castle begins to resemble hell as he has committed a hideous crime. Second, the Porter reminds the audience that bad deeds, such as murder and suicide, are met with punishment in hell. This foreshadows not only Macbeth and Lady Macbeth's coming end, but the promise of punishment in the afterlife for their ambition. It is unclear whether or not this line was the basis for the modern Knock, Knock joke.
"’Tis said they eat each other...."
See in text (Act II - Scene IV)
Shakespeare uses these signs and warnings to foreshadow that more unpleasantness will happen. The eclipse and the story of the owl and falcon to show how nature has become unbalanced as a result of Duncan's murder.
"What's done is done...."
See in text (Act III - Scene II)
Lady Macbeth uses this speech to calm Macbeth's guilty conscious. However, her flippant response to Macbeth's worries, essentially there's no changing what has happened so forget about it, comes back to haunt her when she begins to feel pangs of guilt. Her "what's done is done" later transforms into "what's done cannot be undone," as Lady Macbeth becomes overrun with guilt.
"And damn'd all those that trust them!..."
See in text (Act IV - Scene I)
Even though the last vision upset him, Macbeth still trusts the rest of the information the three witches gave him. Therefore, by making this statement, Macbeth doesn't realize that he is actually damning himself, foreshadowing what happens to him as a result of his belief in the witches' prophesies.
"Give me my armor..."
See in text (Act V - Scene III)
Macbeth is a good soldier and a good leader of soldiers, but he is an incompetent king. Shakespeare seems to be showing his audience that Macbeth feels more comfortable in armor than in a royal robe. Recall how in the previous scene Angus said Macbeth's title did not fit him. The armor gives Macbeth some feeling of security because it "fits" him, physically and temperamentally. It also foreshadows the decisive battle that is about to take place.