In its simplest sense, dramatic irony occurs when the audience knows something that one or more characters in the play don't know. It's important to remember that dramatic irony occurs in "real time," not in retrospect.
In act 4, scene 1, an apparition tells Macbeth that he cannot be defeated until "Great Birnam wood to high Dunsinane hill / Shall come against him" (4.1.104-105).
In act 5, scene 4, which takes place near Birnam Wood, Malcolm tells his soldiers to conceal themselves behind branches so Macbeth and his forces won't know how many soldiers are aligned against them.
MALCOLM. Let every soldier hew him down a bough,
And bear't before him: thereby shall we shadow
The numbers of our host, and make discovery
Err in report of us. (5.4.6-9)
In act 5, scene 5, a Messenger comes to Macbeth to tell him that he's seen Birnam Wood moving. At first, Macbeth refuses to believe the Messenger and threatens him with death if he's not telling the truth.
Macbeth never actually sees the "moving grove," as the...
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