How could I summarize the plot of Every Day by David Levithan in five to seven sentences?

Because the plot of a story has exactly six elements, your question sets up a perfect scenario for a tiny plot summary of Every Day.  The plot always begins with the exposition, when the reader learns about the character and the setting.  During this part of the story, the reader learns about A, who lives in a body only for 24 hours before switching to another one.  Next comes the inciting incident (that some call the conflict):  the problem in the story that needs to be solved.  This particular incident here is when A falls in love with Rhiannon when he inhabits the body of Justin.  This is followed by the rising action, when the tension begins to mount.  Many things happen in the rising action of Every Day.  In fact, in almost every moment of the rising action, A is trying to reach Rhiannon through another body.  Meanwhile, he is foiled a bit by other bodies that he has previously inhabited. The tension continues until it reaches the story's climax:  the height of the tension.  There is no doubt that the climax of the story is when Rhiannon and A figure out that they cannot have a regular relationship.  The climax is followed by the falling action, when things begin to return to normal:  A knows the result of their climactic decision is best for Rhiannon.  By the time of the resolution, the original conflict has been solved in some way.  In fact, A takes his "new" body (Katie) on an adventure in order to disappear.  If you apply these ideas to your assignment for Every Day, it should be easy to write five to seven sentences having to do with the plot.