A motif is a reoccuring object or idea in a work of literature. It's usually a smaller idea than a theme, and there can be several different ones throughout the same piece. Check out the following video:
9. What do you think a main motif in Oedipus Rex is going to be?
Sometimes in literature, an author uses physical or literal things to also represent something bigger, something that goes beyond the literal -- usually an idea or theme that turns out to be very important for the story or narrative. When that happens, the thing in question becomes a symbol.
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11. Name at least three physical things that function as symbols in this play: