There are many literary devices in this text. One of the most powerful is repetition. The author uses words over and over, such as “mad” and uses the same word repeatedly in one sentence. Consider this sentence, for example.
TRUE!—NERVOUS—VERY, very dreadfully nervous I had been and am; but why will you say that I am mad?
The repetition of very and nervous leads us to believe that yes, the narrator is mad!
Another technique used is suspense. This is powerful in the story because much of the effect is psychological. The most suspenseful moment is the climax, when the narrator is showing the police around in the room where he has buried the old man under the floorboards. However, you’ll notice suspense is used early on. In the beginning of the story, the narrator describes in painstaking detail how he opened the door minutely bit by bit to stare at the old man’s eye while he was asleep.
And every night, about midnight, I turned the latch of his door and opened it—oh so gently! And then, when I had made an opening sufficient for my head, I put in a dark lantern, all closed, closed, that no light shone out, and then I thrust in my head. Oh, you would have laughed to see how cunningly I thrust it in! I moved it slowly—very, very slowly, so that I might not disturb the old man's sleep. It took me an hour to place my whole head within the opening so far that I could see him as he lay upon his bed.
A third technique used is irony. Irony comes in many types, but in general it is something that you don’t expect. For example, take a look at this statement.
I was never kinder to the old man than during the whole week before I killed him.
This is ironic because the narrator is being kind to the old man preparing to kill him. It demonstrates his insanity.