Poe creates tension in several ways. First, his use of the first person narrator helps build suspense. For example, right away our narrator address the reader, "True! -- nervous -- very, very dreadfully nervous I had been and am; but why will you say that I am mad?" Here the reader is drawn right in to the story and its tension. The reader must decide is the man really mad? Is he reliable? What can be believe and what might be lies? All of these put the reader on edge.
Next, Poe's syntax, or word choice, is another way he creates tension. It is written as if the narrator is confessing to us. His uses of repetition and asides again draws the reader in and helps build suspense.
Poe also uses plot structure to create tension. Look at the scene where our narrator spies on the old man at night. Our narrator slowly opens the door a crack and each night after a little farther until the light falls on the man's face. Then when he finally is about to enter, after the eighth night, the man wakes up and startles our narrator. What tension!
Also, look at the methodical nature with which the narrator goes about covering up his crime. That builds tension.
Finally, look at the narrator's arrogance. How he seats himself right over the old man's body buried in his floor boards. The reader cannot help but wonder will he get away with it? Will he crack? Is he insane? What will happen?
In "Tell-Tale Heart" Poe creates tension through his first person narrator. This narrator is obviously deranged from a nervous breakdown and, as such, his behavior cannot be anticipated by the reader. Thus, there is an anxiety of sorts that develops in the reader who wonders what the narrator will do.
Perhaps the greatest tension is created by the words of the narrator. At the outset he frantically declares,
True! Nervous-very nervus, dreadfully nervous I had been and am. But why will you say that I am mad? The disease had sharpened my senses...
These words are the classic case of "The lady doth protest too much" and the reader wonders why the narrator tries to explain himself. What has he done? More tension is created as Poe's narrator wavers in his actions and thoughts, stating his love for the old man, while at the same time expressing horror for the "vulture eye." The narrator continues the unexpected as he explains his bizarre actions as wise: "Ha! Would a madman have been so wise as this?"
His calm disposal of the body, in contrast to the frenetic exclamations of the narrator seem to reignite the tension felt in the rising action of the plot. Finally, instead of an expected resolving of the plot, Poe's narrator reveals his deadly crime.
For the most part your question has been answered at the first link below in a previous response.
Here are some quotes that aid in building tension.
"True! -- nervous -- very, very dreadfully nervous I had been and am; but why will you say that I am mad?" The narrator has the reader wondering about his sanity and why is he so nervous. What crime did he commit?
Later when he is opening the door to spy on the old man, he observes: "So I opened it -- you cannot imagine how stealthily, stealthily -- until, at length, a single dim ray, like the thread of a spider, shot from out the crevice and full upon the vulture eye." Will he get caught? Will the old man wake up? What will the narrator do to the old man? Poe's diction is expertly manipulating the reader.
Finally, at the end of the story - "O God! what could I do? I foamed -- I raved -- I swore! I swung the chair upon which I had been sitting, and grated upon the boards, but the noise arose over all and continually increased" The man has obviously been driven mad by the beating of the heart, which could be interpreted as his guilty conscious or even his own heartbeat.
The beating of the heart, whether real or imagined, gets louder and louder as the narrator's hysteria increases. This crescendoing effect underlines the heightening tension in the story line (much as a drum roll...) until the cymbal crash ("...hideous heart!" ) at the end.
He creates suspense in the readers minds because we are unaware of whether the narrator is male or female. The use of 1st person confuses the reader, and therefore makes the reader jump to conclusions that the narrator is a man, which can be classed as stereotypical.=)
Poe draws out the description of police officers' arrival and search, building suspense over whether the narrator's crime will be discovered or confessed.