"The Story of an Hour" utilizes foreshadowing to increase the tension leading up to Mrs. Mallard's tragic end.

In the opening line, readers are made aware that Mrs. Mallard, the protagonist of the story, has "heart trouble." This foreshadows her unexpected reaction to seeing her husband alive in the end. Because of this "heart trouble," Mrs. Mallard collapses in shock; her cause of death is identified as heart disease.

The identification of Brently Mallard's body is rushed and incomplete. After a "railroad disaster," Brently's name quickly appears at the top of those killed in the accident. After a second telegram arrives at the newspaper office, a man rushes to give Mrs. Mallard the news. This man had not physically seen the body and rushed to Mrs. Mallard so that he could arrive before someone who might be "less careful" with the information. Ironically, his lack of care in knowing with certainty that Brently Mallard had died foreshadows the ending when Brently arrives home, completely safe.

Mrs. Mallard's reaction to her husband's death foreshadows her unlikely emotions, which she hides until she is completely alone. She "did not hear the story as many women have heard the same." Instead of taking a moment to process the news, Mrs. Mallard immediately begins weeping with "wild abandonment." Though her tears are interpreted as grief in that moment, her instantaneous acceptance of the news foreshadows the tears which later come in private—tears of joy.

These examples of foreshadowing create a growing tension as Mrs. Mallard embraces joy in her newfound freedom, though it is taken from her in the end.

Through foreshadowing, writers provide information or details within the story in preparation for future plot points. You can observe several examples of this technique in "The Story of an Hour."

Probably the most immediately apparent of these examples can be found in the very first line:

Knowing that Mrs. Mallard was afflicted with heart trouble, great care was taken to break to her as gently as possible the news of her husband's death.

With this line, Kate Chopin establishes Mrs. Mallard's heart condition, a detail that proves critical, given that the story ends with her sudden death. For that plot twist to land with any effectiveness, however, the writer needs to establish its possibility in the back of readers' minds. The alternative is to run a serious risk of the ending feeling contrived.

While this opening statement is probably the most important example of foreshadowing to be found in this story, it is not the only one. Another example can be seen in Mrs. Mallard's physical description, where Kate Chopin writes,

She was young, with a fair, calm face, whose lines bespoke repression and even a certain strength.

Note Chopin's specific choice of phrasing in this sentence, with her use of the words repression and strength. From the word choice alone, Chopin gives the impression of a woman who has suffered in life and has endured that suffering, an impression that is intended to set the stage for the epiphany that follows as Mrs. Mallard realizes the degree to which marriage, as an institution, is fundamentally oppressive to her spirit.

One final example can be found in how the news of her husband's death was relayed:

Her husband's friend Richards was there, too, near her. It was he who had been in the newspaper office when intelligence of the railroad disaster was received, with Brently Mallard's name leading the list of "killed." He had only taken the time to assure himself of its truth by a second telegram, and had hastened to forestall any less careful, less tender friend in bearing the sad message.

Note, however, that Richards "hastened" to tell Mrs. Mallard the news. Though it appears he attempted to verify the news, Richards himself was not a witness to the accident, and he rushed ahead to report the news of the tragedy without too much of a follow-up. These details are important ones in shaping the story's ending, when we find out that these initial reports had actually been mistaken, with Mallard having been nowhere near the area of the accident at all.

Foreshadowing exists in this story in two different aspects.  The first is foreshadowing of Louise Mallard's unique and ironic reaction to the news of her husband's death. The next is foreshadowing of her unfortunate demise at the end of the story.

For foreshadowing of her feelings of joy at the news of her husband's death, look to the following instances.  The first is when Chopin describes Louise's beautiful face "whose lines bespoke repression." Chopin mentions that her face reflected "repression"; so, the woman felt repressed in her role as a wife--indicating that she might not be sad to be relieved of that role.  Then, as Louise looks out the window after hearing of the tragedy of her husband's death, she sees

"the tops of trees that were all aquiver with the new spring life. The delicious breath of rain was in the air. ...and countless sparrows were twittering in the eaves."

One would expect a stormy sky, lightning and thunder, but Chopin uses the setting to foreshadow the freedom that Louise feels at her husband's death; her emotions, like the weather, are sunny, happy, and full of promise and new life, foreshadowing her coming elation.  As she continues to think about her husband, Chopin writes that Louise could sense a feeling coming and "was waiting for it, fearfully. What was it?" This foreshadows the feeling of elation that she will feel at her husband's death; a fearful emotion because of how inappropriate it is, yet how wonderful freedom feels to her.  From here on out, the theme is obvious as Louise basks in feeling "Free!  Body and soul free!"

For foreshadowing of her death at the end, look to the beginning where Richards and her sister were afraid to tell her of the death of Brently, because she had heart troubles.  If they were worried about her heart acting up at the news of his death, it makes sense that it would act up when he unexpectedly arrives.

I hope that those examples help; good luck!