Apples
Apples are used in the bobbing game during the Halloween party at the start of the novel. Apples are symbolic of temptation and sin. In fact, Joyce is killed in a vat of apples meant for the bobbing contest, showing the sinful nature of apples.
Knives
Knives are not only symbolic of the way that Joyce and Olga were murdered; they are symbolic of a human being's ability (and, oftentimes, willingness) to do bad things to other people.
Poirot's foreignness
Hercule Poirot is often looked at as an outsider in countries outside of Belgium, where he was born. One of the key aspects of his character is his ability and willingness to forgive people for slights against him and those around him. Throughout the novel, he forgives countless people, creating a motif. It's because of this motif, after all, that Poirot is able to uncover so many of the community's secrets.
Candles
Candles, which appear frequently throughout the novel, symbolize clarity. Much of the novel is set at night, and candles give light in dark times. They allow people who would otherwise be lost to be clear about everything going on around them.
Fall
The novel is set during the fall and symbolizes decay and destruction. As the season transitions from summer to fall and from fall to winter, people die, and others have their lives changed. For instance, Joyce is killed during the fall.
