Myths are generally considered to be traditional stories whose importance lies in their significance, like the myth of the Fall in Eden; whereas legends can be merely famous deeds, like the legend of Davy Crockett. In common usage “myth” usually implies fantasy. Enrico Caruso was a legendary tenor, but Hogwarts is a mythical school. Legends may or may not be true.
But be cautious about using “myth” to mean “untrue story” in a mythology, theology, or literature class, where teachers can be quite touchy about insisting that the true significance of a myth lies not in its factuality but in its meaning for the culture which produces or adopts it.
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