Question: What is the difference between having multiple plots vs just a single plot? I have to decide whether Bridge to Terabithia has multiple or single plots but can't when I don't even know how to tell the difference.
The one simple way is to notice if the story is told from the point of view of more than one character. While most of the scenes will be told from the point of view of the main character, there may be scenes presented from a different point of view (possibly because the main character isn't present for those scenes).
If the scenes with the other point-of-view character form an independent story, that is a subplot.
Unfortunately, things get confusing if the main character is also involved in subplots, but there are clues to watch for.
If those subplot scenes have little to do with the main plot but, taken on their own, tell a complete story; that is, they revolve around a different goal or problem, and that endeavour has its own arc and resolution, then you have a subplot.
Subplots can revolve around...
* A problem that illustrates an important theme from a different angle.
* The evolution of a relationship between two minor characters.
* A side quest or secondary problem.
Here's another test...
If you can remove from the novel all the scenes you suspect are part of are a subplot and the main story still makes sense and is complete, the subplot is a true subplot.
If you were to string together all the scenes you think are part of a subplot, and exclude the rest of the novel, and these scenes tell a complete story on their own, they are a true subplot.
Hope that helps.