How is "I forget your arms." (said Fortunato) "A huge human foot d'or in a field azure; the foot crushes a serpent rampant whose fangs are imbedded in the heel." (said Montressor) an example of foreshadowing

It shows the coat of arms' significance in avenging anyone who's betrayed or humiliated them. And similarly, Montresor takes revenge on Fortunato by walling him in, thus the arms' motto foreshadows Fortunato's fate.

Explanation:

Foreshadowing is the use of words in such a way that it gives a feeling of familiarity or knowledge about what will happen in the future. It provides a hint or a clue on what will happen in the coming scenes.

In the given quotes or lines from Edgar Allen Poe's "The Cask of Amontillado", the foreshadowing can be understood as these lines present the readers an insight into what the inner meanings of the family's coat of arms represent. The Montresor family's coat of arms about "a huge human foot . . .[crushing] a serpent rampant whose fangs are imbedded in the heel" shows how the Montresor believes in crushing anyone who makes fun, humiliate, wronged, or betrayed them. This means that Montresor will also take revenge on Fortunato for all the 'embarrassment' and insults he had endured.

This foreshadows Fortunato's future suffering or death at the hands of Montresor.  

"the day we found the sharks' teeth" i would say highlight the element of the plot as it sets its boundaries and centers the adventures of them being on a boat and cleaning spit of sand.

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