Which element in a story can the reader use to understand the story's theme?

The plot.

Hope this helps. :)

Explanation:

The plot is used to describe the events that make up the story. The plot is basically the foundation of a story, around which the characters and settings are built.

type of error:

the error in the sentence is a "dangling modifier". a dangling modifier is a phrase, word or clause that "dangles" because there is not target to modify. in this case, "after mopping the floor" is the dangling modifier since it has no word to modify. the noun "house" cannot mop the floor; therefore there is a mistake in the writing of the sentence.