Which nonlinear element of plot describes something that happened

earlier than the main action of the story?

Flashback

Explanation:

Flashback is considered a method inserted in the story to show happened earlier than the main action of the story. It is used to tell the story that happened before the main one or to give background information.

This is a technique used to show the events but not following the chronological order strictly. This method of flashback makes the story more vivid and readers are getting more involved in the story.